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.

“The Durability of Poly(Methyl Methacrylate) Lenses 

Used in Concentrating Photovoltaic Technology”

ATLAS/NIST Workshop on Photovoltaic 

Materials Reliability

2011/10/27 (Thurs)

12:00 – 12:35 pm 

Green room (Administrative Bldg)

1National Renewable Energy Laboratory

2Colorado School of Mines

3Binghamton University, SUNY

*Presenter:

David.Miller@nrel.gov

NREL/PR-5200-53361

David C. Miller1*, Joseph D. Carloni2,3, Joel W. Pankow1, Erica L. Gjersing1, 

Bobby To1, Corinne E. Packard2, Cheryl E. Kennedy1, and Sarah R. Kurtz1
 

 

 

 

 

 

The Use of Fresnel Lenses in CPV
 •Concentrating Photovoltaic (CPV) modules use cost effective optics ($)
 

to focus light onto high efficiency (η=43%) multijunction cells ($$$$$)
 

• Many present CPV implementations use refractive optics (Fresnel lenses) 

Implementations of CPV , including refractive and reflective, shown relative to flat-panel technology 

Miller et. al., Optical Engineering, 50, 2011, 013003.
 

Miller et. al., Solar Energy Materials and Solar Cells, 95, 2011, 2037.
 

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The Use of Fresnel Lenses in CPV
 •Concentrating Photovoltaic (CPV) modules use cost effective optics ($) 

to focus light onto high efficiency (η=43%) multijunction cells ($$$$$) 

• Many present CPV implementations use refractive optics (Fresnel lenses) 

Implementations of CPV , including refractive and reflective, shown relative to flat-panel technology 

•Monolithic PMMA is the most cost 

effective, lightweight lens 

implementation (vs. silicone on glass) 

•The durability (expected life) for lenses is 

unknown for the CPV application 

Miller et. al., Optical Engineering, 50, 2011, 013003.
 

Miller et. al., Solar Energy Materials and Solar Cells, 95, 2011, 2037.
 

Cross-sectional schematics of monolithic PMMA and SoG lenses
 

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Purpose- and Details- of Screen Test at NREL
 •Literature ⇒ studies initiated ≥20 years ago 

•Goal here: characterize the durability of a broad range of 

contemporary specimens subject to indoor HAST 

DESCRIPTION SPECIMEN TYPES 

stock (unpatterned) 11 

linear focus lens 1 

spot focus lens 8 

veteran (fielded) lens 5 

•Veteran specimens on tracker in desert 

site, seldom cleaned (8, 22, 27 yrs) 

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Purpose- and Details- of Screen Test at NREL
 •Literature ⇒ studies initiated ≥20 years ago 

•Goal here: characterize the durability of a broad range of 

contemporary specimens subject to indoor HAST 

2 5/8 ” 

1 3/4 3x ”

DESCRIPTION SPECIMEN TYPES 

stock (unpatterned) 11 

linear focus lens 1 

spot focus lens 8 

veteran (fielded) lens 5 

•Veteran specimens on tracker in desert 

site, seldom cleaned (8, 22, 27 yrs) 

Test specimens (4.4 x 6.7 cm2)
 

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Purpose- and Details- of Screen Test at NREL
 •Literature ⇒ studies initiated ≥20 years ago 

•Goal here: characterize the durability of a broad range of 

contemporary specimens subject to indoor HAST 

2 5/8 ” 

1 3/4 3x ”

Now also “round 2”: (38) PMMA , SoG specmens. Indoors and FL, AZ, CO. 

•Veteran specimens on tracker in desert 

site, seldom cleaned (8, 22, 27 yrs) 

DESCRIPTION SPECIMEN TYPES 

stock (unpatterned) 11 

linear focus lens 1 

spot focus lens 8 

veteran (fielded) lens 5 

Test specimens (4.4 x 6.7 cm2) 

•Test instrument: ATLAS Ci4000 Weather-ometer 

(Xenon-arc lamp @ 2.5x UV suns. Chamber @ 60°C/60%RH) 

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Details of the NREL Screen Test 

•Measurands: 

Periodic 

optical appearance (photograph) 

optical transmittance (hemispherical & direct)
 

mass
 

contact angle (sessile drop, H2O on 1st surface)
 

“End of life” 

haze (from direct transmittance) 

prism facet geometry (lenses: section then SEM) 

surface morphology (SEM or AFM) 

indentation (Berkovich hardness, toughness) 

XPS (surface chemistry, before & after cleaning) 

fluorescence spectroscopy 

MALDI (Mw) 

NMR (bulk chemistry) 

•Test schedule: 

0, 1, 2, 4, 6, 12, 18, 24, 30, 36 months 

≥8 acceleration factor (irradiance and 24 hour operation) 

pull 1 of replicates every 12 months 

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Summary of Trends Observed to Date
 

Transmittance: 

•Reduced with age, especially purple & blue wavelengths 

•Effects similar for degradation & soiling 

•H2O affects IR wavelengths only (e.g., 1415, 1910, 2700 nm) 

Contact Angle: 

•Reduced with age (66 ° → 57 ° [12 months] → 43° [veteran]) 

•Significant restoration with cleaning 

Mechanical Fracture: 

•Radial cracks on injection molded lens 

Surface Roughness: 

•Solid erosion, scratches, embedded material 

@ 6 months: Miller et. al., Proc. SPIE, 2010, 7773-02.
 

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2 Different Damage Morphologies Have Emerged!
 Transmittance characteristics and visual appearance of the two damage morphologies 

“hazy” “yellow” 

•Specimens have a hazy (scattering) or yellow (attenuating) appearance
 

•Both examples are “solar grade” material, same manufacturer 

•What are the significance and cause? 

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Electrical Interconnection in Multijunction and Si CPV Cells 

•Because 3J cells are connected in series, the characteristics of the 

optical system have increased importance 

•Optical degradation & soiling are critical reliability issues 

Nann and Emery, Solar Energy Mats Solar Cells, 27, 1992, 189-216 http://en.wikipedia.org/wiki/Multijunction_photovoltaic_cell 

Green et. al., Prog Photovoltaics, 18, 2010, 346-352. (2011/10/10) 

External quantum efficiency for terrestrial 3J and c-Si cells Cross-section of typical commercial 3J cell 

Critical characteristics for common cell technologies 

Circuit schematic for typical 3J cell 

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YI Distinguishes the Aged PMMA Formulations
 

YI: ASTM E313 & E308 

[D65 source, 10° observer] 

•A broad variety of results follows 

for the sheet specimens 

• 5/11 shown were CPV or PV 

marketed materials, including 

(2) “hazy” and (3) “yellow” 

YI determined from raw hemispherical transmittance measurements
 

(proportional to visual appearance) for sheet specimens.
 

Veteran lens fielded 8 years
 

•YI correlates well with transmittance!
 

(i.e., expected module performance)
 

•Single quantitative figure of merit 

YI vs. T (300-650 nm or 300-1220 nm) 

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Differences in the Optical Transmittance of PMMA & MMA
 

Transmittance of PMMA vs. the absorbance of MMA for h=3.2 mm 

•The transmittance of (unaged) PMMA polymer follows from MMA monomer 

•Differences: T at 265 nm, absorption from 300-380 nm in PMMA 

• MMA & unformulated PMMA have similar spectra 

⇒ differences from additives, co-polymer 

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Contact Angle Is Affected By Age 

•Contact angle relates to the 

accumulation and retention of 

particulate matter 

•Contact angle decreased with 

time for hazy & yellow specimens 

•Both similarly restored when 

cleaned at 6 months 

•Decreasing Θ, with no distinction 

between h & y specimens 

Contact angle for hazy and yellow specimens. 

Helpful: measure after desiccating average Θ shown for sheet specimens 

veteran lens fielded 8 years 

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Surface Features of the Indoor Aged Specimens
 (a) unaged reference, (b) cracked, (c) porous, and (d) roughened surface morphologies at 12 months cumulative aging 

Ra±Rrms Rpv Ra±Rrms Ra±Rrms Rpv R Ra±Rrms Rpv pv

6±11 nm 224 24±31 793 27±37 1,085 30±39 649 

30 μm 

5±6 nm 51 19±24 524 23±31 410 

Rpv 

(b) (c)

Ra±Rrms Ra±Rrms Rpv Ra±Rrms Rpv 

 KIC 

2

ao = π  

20 μm Yσf 

•Cracking and surface porosity are evident 

•A defect size of 35-75 μm, however, is implied for unaged PMMA from 

its fracture strength (70-105 MPa) 

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(a) (b) (c) (d) 

(a) 

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unaged 12 months 

Surface Roughness Increases With Aging
 hazy yellow 

Representative isometric representation of hazy & yellow specimens 

•Roughness increased significantly (10x) with age 

SPECIMEN 

SET HISTORY Ra {nm} Rrms {nm} 

hazy unaged 3.1 4.4 

hazy aged 26.0 38.8 

yellow unaged 2.6 3.5 

yellow aged 21.9 28.1 

•No overt distinction, hazy vs. yellow 

•Outdoor deployment also includes surface 

abrasion (solid erosion) & particle implantation 

•Increased roughness should scatter light 

(increase haze) and reduce contact angle Summary of AFM measurements 

•Crazing/cracking could relieve stress 

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Surface Roughness Increases With A

hazy yellow

unaged 12 months 

Representative SEM images (nominally 15 kx)

2

m

Morphology qualitatively similar in SEMSome irregular features (including spikes) for unaged m

Morphology porous (varied

) across aged

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“Nanoindentation” Quantifies Mechanica

Hol

SEM image of Berkovich tip 

Load/displacement profile for PMMA



A prescribed diamond tip is pressed into the specimen

Elastic recovery of strain occurs at the instant of unloa

The key tribological characteristics (E,H) can be determ

from the load/displacement data

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Indentation Distinguishes the Aged PMMA Formulations
 

•Specimens potted & x-sectioned for indentation 

(150 x 5 matrix) in the thickness direction 

•Little effect observed near the surface
 

•Little effect observed for hazy (aged vs. unaged) 

•E affected for yellow (aged) through the thickness 

•Degradation and/or ∆stress may explain the result 

Indentation results (elastic modulus and Berkovich hardness) 

Schematic 

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Mass Loss Resulting From Aging
 

•Resembles asymptotic diffusion process. Degradation products? Additives? 

•∆m of hazy and yellow specimens not readily distinguished 

•∆m similar for veteran and contemporary specimens 

•∆m from lost volatile content would be proportional to cumulative damage 

•Critical: measure after desiccating 2 weeks 

Specimen mass, relative to that at month 1. 

Average ∆m shown for sheet specimens. 

Veteran lens fielded 8 years. 

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Thermometric Gravitational Analysis (TGA)
 

Does Not Distinguish the Hazy and Yellow Formulations
 

SPECIMEN ATMOSPHERE 

T, 

0.5% 

mass loss 

{°C} 

T, 

50% mass 

loss 

{°C} 

FREQUENCY 

FACTOR: 

c1, avg 

(s-1) 

Ea, avg±SD 

(kJ/mol) 

unformulated O2 201 256 2.1E+07 108±8 

unformulated N2 185 328 4.7E+13 194±44

hazy O2 256 288 8.3E+07 115±6 

hazy N2 258 338 7.7E+15 223±15 

yellow O2 250 281 2.0E+07 108±6 

yellow N2 257 338 5.0E+16 232±15 

− Ea  kr = c1 exp

 RT 

•O2 facilitates thermal degradation 

Table and figure: modulated TGA results for the specimens 

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Thermometric Gravitational Analysis (TGA)
 

Does Not Distinguish the Hazy and Yellow Formulations
 

SPECIMEN ATMOSPHERE 

T, 

0.5% 

mass loss 

{°C} 

T, 

50% mass 

loss 

{°C} 

FREQUENCY 

FACTOR: 

c1, avg 

(s-1) 

Ea, avg±SD 

(kJ/mol) 

unformulated O2 201 256 2.1E+07 108±8 

unformulated N2 185 328 4.7E+13 194±44

hazy O2 256 288 8.3E+07 115±6 

hazy N2 258 338 7.7E+15 223±15 

yellow O2 250 281 2.0E+07 108±6 

yellow N2 257 338 5.0E+16 232±15 

− Ea  kr = c1 exp

 RT 

•O2 facilitates thermal degradation 

•Formulation improves stability 

(co-polymer, additives) 

Table and figure: modulated TGA results for the specimens 

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Thermometric Gravitational Analysis (TGA)
 

Does Not Distinguish the Hazy and Yellow Formulations
 

SPECIMEN ATMOSPHERE 

T, 

0.5% 

mass loss 

{°C} 

T, 

50% mass 

loss 

{°C} 

FREQUENCY 

FACTOR: 

c1, avg 

(s-1) 

Ea, avg±SD 

(kJ/mol) 

unformulated O2 201 256 2.1E+07 108±8 

unformulated N2 185 328 4.7E+13 194±44

hazy O2 256 288 8.3E+07 115±6 

hazy N2 258 338 7.7E+15 223±15 

yellow O2 250 281 2.0E+07 108±6 

yellow N2 257 338 5.0E+16 232±15 

− Ea  kr = c1 exp

 RT 

•O2 facilitates thermal degradation 

•Formulation improves stability 

(co-polymer, additives) 

Table and figure: modulated TGA results for the specimens 

•Depolymerization is a high Ea process 

(60 °C in Ci4000 vs. ≥250 °C in TGA) 

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Thermal Decomposition ... Not Suspected Here 

•Unzipping of main chain into methyl methacrylate (monomer) 

•Autocatalytic process (zip length on order of 1000) 

•Significant weight loss (100% vs. minimal for chain scission) 

Miller et. al., Solar Energy Materials and Solar Cells, 95, 2011, 2037.
 

(after Aboulezz and Waters, “Studies on the Photodegradation of Poly(Methyl Methacrylate)”, 1978)
 

•Synergistic effect w/ irradiation (UV)⇒occurs at T<200°C 

•Unlikely to explain distinction between hazy and yellow specimens 

(similar ∆m and Ea characteristics) 

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X-Ray Photoelectron Spectroscopy (XPS) Quantifies Composition
 

1 

2 

•Surface (h≤10 nm) analysis technique 

•Technique distinguishes valence and/or 

chemical state: 

→O=C (carbonyl oxygen) 

is distinct from O-CH3 (ester oxygen) 

Schematic and XPS data:
 

O and C peaks distinguished for clinical PMMA
 

2 1
 

←positive charge/environment negative charge/environment→

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X-Ray Photoelectron Spectroscopy (XPS) Quantifies Composition
 

•Surface (h≤10 nm) analysis technique 

•Technique distinguishes valence and/or 

chemical state: 

→O=C (carbonyl oxygen) 

is distinct from O-CH3 (ester oxygen) 

→CH2 (methylene C) and CH3 (methyl C) 

look the same. But 4c’s! Schematic and XPS data:
 

O and C peaks distinguished for clinical PMMA
 

1 

2 

3 

1 2 

1 

4 

2 1 

1 

4 3 2

←positive charge/environment negative charge/environment→

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X-Ray Photoelectron Spectroscopy (XPS) Quantifies Composition
 

•Surface (h≤10 nm) analysis technique 

•Technique distinguishes valence and/or 

chemical state: 

→O=C (carbonyl oxygen) 

is distinct from O-CH3 (ester oxygen) 

→CH2 (methylene C) and CH3 (methyl C) 

look the same. But 4c’s! Schematic and XPS data: 

O and C peaks distinguished for clinical PMMA →O and C peaks deconvoluted via 

fit model for O1s and C1s 

• ~1 mm2 X-ray spot used here 

1 

2 

3 

1 2 

1 

4 

2 1 

1 

4 3 2

1 

4 3 2 2 1 

←positive charge/environment negative charge/environment→

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XPS Spectra Identify Chemistry Changes (O↑, C↓) 

∆ shape, magnitude O 

unformulated measured separately, shown for reference 

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XPS Spectra Identify Chemistry Changes (O↑, C↓) 

∆ shape, magnitude O 

∆ magnitude C 

unformulated measured separately, shown for reference 

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XPS Spectra Identify Chemistry Changes (O↑, C↓) 

∆ shape, magnitude O 

∆ magnitude C 

•similar trends 

•∆O: from hydroxyl? 

unformulated measured separately, shown for reference 

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XPS: Quantification of Changes at the Surface
 

•Oxygen content increased, carbon decreased
 

COMPOSITION 

(IDENTIFYING PEAK) 

{% at. concentration} 

SPECIMEN HISTORY C (1s) O (1s) N (1s) F (1s) Si (2p) 

unformulated unaged 72.4 27.6 0.0 0.0 0.0 

hazy unaged 73.5 26.2 0.0 0.0 0.2 

hazy aged 62.6 32.0 1.1 1.2 2.9 

hazy aged, cleaned 64.3 32.4 0.0 1.2 2.0 

yellow unaged 71.9 27.8 0.0 0.0 0.3 

yellow aged 61.8 33.7 0.7 1.2 2.5 

yellow aged, cleaned 65.1 31.8 0.2 1.0 1.9 

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XPS: Quantification of Changes at the Surface
 

•Oxygen content increased, carbon decreased 

•N, F, Si evident after aging. Removable by washing 

→N corresponds to removable polar species expected from ∆Θ
 

→N from “blooming” or decomposition of UVA 

(e.g., benzotriazole: C6H4N3H)? 

→F, Si from release paper, typically fluorinated? 

(Maybe from additive... more likely lubricant/mold release) 

COMPOSITION 

(IDENTIFYING PEAK) 

{% at. concentration} 

SPECIMEN HISTORY C (1s) O (1s) N (1s) F (1s) Si (2p) 

unformulated unaged 72.4 27.6 0.0 0.0 0.0 

hazy unaged 73.5 26.2 0.0 0.0 0.2 

hazy aged 62.6 32.0 1.1 1.2 2.9 

hazy aged, cleaned 64.3 32.4 0.0 1.2 2.0 

yellow unaged 71.9 27.8 0.0 0.0 0.3 

yellow aged 61.8 33.7 0.7 1.2 2.5 

yellow aged, cleaned 65.1 31.8 0.2 1.0 1.9 

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The “Mechanics” of Fluorescence
 

•In polymers, illumination can generate optical 

absorption and fluorescence where: 

intensity ∝ lumophore concentration 

wavelength ∝ lumophore composition 

lumophore sites: UVA, impurities, chromophores 

1. Excite at 477.5 nm, measure emission from 487.5-825 nm 

*there is a peak @ 525 nm! 

*avoid harmonics (λh=2λ) e.g., 955 nm 

M and X fluorescence spectra for “yellow” specimen 

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The “Mechanics” of Fluorescence
 

•In polymers, illumination can generate optical 

absorption and fluorescence where: 

intensity ∝ lumophore concentration 

wavelength ∝ lumophore composition 

lumophore sites: UVA, impurities, chromophores 

2. Measure at 525, excite from 280-515 M and X fluorescence spectra for “yellow” specimen 

*there are peaks at 285, 330, and 477.5 motivating emission @ 525 nm 

*iterate for excitation peaks @ 280, 330 nm (emission spectra) 

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The “Mechanics” of Fluorescence
 

•In polymers, illumination can generate optical 

absorption and fluorescence where: 

intensity ∝ lumophore concentration 

wavelength ∝ lumophore composition 

lumophore sites: UVA, impurities, chromophores 

1. Excite at 477.5 nm, measure emission from 487.5-825 nm 

*there is a peak @ 525 nm! 

*avoid harmonics (λh=2λ) e.g., 955 nm 

2. Measure at 525, excite from 280-515 

*there are peaks at 285, 330, and 477.5 motivating emission @ 525 nm 

*iterate for excitation peaks @ 280, 330 nm (emission spectra) 

M and X fluorescence spectra for “yellow” specimen 

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Fluorescence Suggests Additives Distinguish Formulations
 

•Modest fluorescence observed in test 

relative to unaged reference-specimens 

•Different fluorescent spectra were 

observed for (2) “hazy” 

and (10) “yellow” specimens 

•Different spectra suggest change in 

electronic structure ... is there a 

connection to degradation mechanism? 

Emission (M) and excitation (X) spectra for “hazy” specimen 

•In particular the excitation @ 310nm is 

correlates to the action spectrum for 

PMMA identified in the literature 

•No fluorescence is observed for MMA 

from 250-550 nm 

M and X fluorescence spectra for “yellow” specimen 

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The Degradation Mechanism of Photolysis
 

•Dominant mechanism suggested in literature & suspected here
 

(after Aboulezz and Waters, “Studies on the Photodegradation of Poly(Methyl Methacrylate)”, 1978) 

•Random main chain scission by UV (photolysis) ⇒ Mw therefore Tg reduced 

•Tg reduced ↓~5°C after 18 years outdoors 

L.G. Rainhart & W. P Schimmel, SAND 74-0241, 1974. 

• Mw↓ likewise affects mechanical durability: KIC↓⇒σf↓ ... ∂a/∂N↑

•E could be affected if Mw severely compromised 

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Details of Future Work (2nd Round of Study)
 

Goals: 

•Quantify the acceleration factor: aging indoors vs. outdoors (FL, AZ, CO)
 

•Identify the expected life of PMMA in these key locations 

•Understand the degradation mechanism(s) [if lifetime can be improved]
 

Noteworthy specimens:
 

•With & without UV stabilizer (compare to academic literature) 

•Residual monomer (suspected to facilitate degradation) 

•High vs. low Mw (mechanism of photolysis) 

•Fabrication method (cast vs. extruded vs. molded) 

•Co-monomer content (methyl acrylate and ethyl acrylate) 

•Polycarbonate (benchmark a competing material) 

•SoG (benchmark a competing technology) 

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Summary of Results 

Weathered PMMA for CPV: 2 specimen morphologies have emerged
 

Not distinguished by: 

•surface morphology (roughness increased in both) 

•contact angle (decreased for both) 

•mass (decreased for both) 

•thermogravimetry (depolymerization not suspected) 

•XPS (O↑ [hydroxyl formed], C↓; removable N, F, Si) 

Distinguished by: 

•transmittance & YI (“hazy” vs. “yellow”) 

•indentation (modulus decreased for yellow) 

•fluorescence (distinction motivated by additives) 

Consistent with random chain scission for formulated PMMA 

(will verify contributing factors) 

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Acknowledgements 

•NREL: Mike Kempe, Lynn Gedvilas, Daryl Myers, John Pern, Dan Ruddy, Matt
 

Beach, Christa Loux, Marc Oddo, Bryan Price, Kent Terwilliger, Robert Tirawat
 

•Others: Ralf Leutz, Hans Philipp Annen (Concentrator Optics Inc.),
 

John Wiedner, Michael Longyear, Scott Steele
 

(Arizona Public Service, S.T.A.R. facility)
 

This work was supported by the U.S. Department of Energy under Contract No.
 

DE-AC36-08GO28308 with the National Renewable Energy Laboratory.
 

This material is based upon work supported by the National Science Foundation 

through the Renewable Energy Materials Research Science and Engineering 

Center under the Grants DMR-0820518 and DMR-1063150 

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Nuclear Magnetic Resonance (NMR): 13C Data
 

Close-up of C region 

Close-up of C=O region 

•Solvent: deuterated chloroform 

•hazy: -appears- δ shifted right (smaller ppm); 

yellow: no δ shift 

No real shift for hazy, rather...
 

accuracy of the lock to the marker species
 

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Nuclear Magnetic Resonance (NMR): 1H Data 

•CH2, CH3 region: random tacticity 

•no major new peaks: no cross-linking 

•hazy only: additive peak @ 4.05 ppm 

•h & y: additive peak @ 3.75 ppm lost by aging 

•h & y: @1.6 ppm, lost w/ aging 

from formulation additive 

•y: broader peaks suggest shorter chains 

Close-up of CH2, CH3 region 

Close-up of O-CH3 region 

? 

(1.6) 

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