THE DURABILITY OF POLYMERIC ENCAPSULATION

MATERIALS FOR CONCENTRATING PHOTOVOLTAIC SYSTEMS

David C. Miller 1*, Michael D. Kempe 1, Kenji Araki 2, Cheryl E. Kennedy1, and Sarah R. Kurtz1 *David.Miller@nrel.gov

1National Renewable Energy Laboratory (NREL), 1617 Cole Blvd., Golden, CO, USA 80401

http://www.nrel.gov/pv/performance_reliability/ 2 Daido Steel Co., Ltd. 2‐30 Daido‐cho, Minami, Nagoya 457‐8545, Japan

Presented at the PV Module Reliability Workshop, February 16‐17, 2011,  Golden, Colorado

Introduction

Polymeric encapsulation materials are typically used in concentrating photovoltaic (CPV) modules to

protect the cell from the field environment. Because it is physically located adjacent to the cell, the encapsulation

is exposed to a high optical flux, often including light in the ultraviolet (UV) and infrared (IR) wavelengths. The

durability of encapsulants used in CPV modules is critical to the technology, but is presently not well understood.

This work seeks to identify the appropriate material types, field‐induced failure mechanisms, and factors of

influence (if possible) of polymeric encapsulation. These results will ultimately be weighed against those of future

qualification and accelerated life test procedures.

Polymeric encapsulation materials are typically used in concentrating photovoltaic (CPV) modules to

protect the cell from the field environment. Because it is physically located adjacent to the cell, the encapsulation

is exposed to a high optical flux, often including light in the ultraviolet (UV) and infrared (IR) wavelengths. The

durability of encapsulants used in CPV modules is critical to the technology, but is presently not well understood.

This work seeks to identify the appropriate material types, field‐induced failure mechanisms, and factors of

influence (if possible) of polymeric encapsulation. These results will ultimately be weighed against those of future

qualification and accelerated life test procedures.

Background

Test specimens, including: ethylene‐co‐vinyl acetate (EVA), polyvinyl butyral (PVB),

poly(ethylene‐co‐methacrylic acid metal salt) (ionomer), polyethylene and polyoctene copolymer (“polyolefin”,

PO) thermoplastic polyurethane (TPU) poly(dimethylsiloxane) (PDMS) and poly(phenyl‐methyl silane) (PPMS)

Experimental Results

Discovery experiments using EVA and PDMS specimens were conducted to become familiar with the CPV

environment and to understand the effect of contamination embedded in the specimens. Catastrophic failure

(thermal runaway followed by material decomposition) may be quickly observed (within minutes or days) in CPV

modules (geometric concentration  500x), if no homogenizing optic is used. The optical concentration at

localized sites within the focus of a single optical element can easily be 1‐3 orders of magnitude greater than the

nominal geometric concentration. Such localized hot points are believed to have facilitated the destruction of EVA

and PDMS specimens.

Soil, aluminum filings, polymer filings (black polyethylene), and voids (bubbles) were intentionally added to

separate sets of EVA and PDMS specimens. Soil, aluminum, and polyethylene (PE) facilitated the thermal

decomposition of EVA within 2, 31, and 2 days respectively. Like the earlier specimens, indicators of significant

stress (including voided regions, localized delamination, and cracked glass) were observed in failed specimens.

Only PE facilitated the thermal decomposition of PDMS; however, the soil and aluminum filled specimens became

PO), thermoplastic polyurethane (TPU), poly(dimethylsiloxane) (PDMS), and poly(phenyl‐methyl silane) (PPMS) cracked within 2 days in the field No primer was applied to the contamination which may have facilitated the

were formed between quartz slides. (Some of the test materials come from the existing flat‐panel PV field and

were not specifically formulated for use in CPV. Hydrocarbon formulations often contain a UV stabilization system

whereas the silicones typically do not). Hydrocarbon‐based materials were laminated to be 0.5‐mm thick (as

popular in existing film products) whereas silicones were molded to be 5.0‐mm thick (to distinguish between

these exceptionally optically transmitting materials). Specimens were situated between a secondary optic and

then inserted in a contemporary CPV module product (nominally 500x concentration). The module uses a domed

Fresnel lens as its primary optical element, and is mounted on a tracker in Golden, CO. Measurements (including

hemispherical transmittance, direct transmittance, mass loss, and visual appearance) were/will be performed

periodically (0, 1, 2, 4, 6, 12, 18, 24, 30, 36 ... months). The initial results (for 6 months cumulative aging in the

field) are summarized here.

cracked within 2 days in the field. No primer was applied to the contamination, which may have facilitated the

cracks in the PDMS. While voids reduce the optical transmittance, the specimens with bubbles did not appear

changed after 116 consecutive days in the field.

ABOVE: Photographs of representative (a) hydrocarbon and (b) silicone test specimens, are inserted (c) behind a secondary optic element and then

(d) into a CPV module. The tracker‐mounted module can accommodate 25 separate specimens (no duplicate formulations are used here).

Environmental conditions at the test site are

logged using a plethora of instruments, including

module‐located thermocouples. Instruments not

immediately present at the Outdoor Test Facility

(OTF) are supplemented by those at the Solar

Radiation Research Laboratory (SRRL), located on

ABOVE: The visual appearance of soil‐contaminated EVA [(a)‐(c)], soil‐

contaminated PDMS [(d)‐(f)], aluminum‐contaminated EVA (g) & (h), and

aluminum‐contaminated PDMS (i) & (j). Discoloration and voiding

(arrow) are evident in (b). The failed EVA specimens contain a voided

central region, surrounded by charred, decomposed, and degraded

material (c) and (h). The PDMS was prone to cohesive failure (crack

formation, [arrows in (e), (f), and (j)]).

The formal experiment is presently examining (6)

EVA, (2) PVB, (2) ionomer, (1) PO, (1) TPU, (11) PDMS,

and (2) PPMS specimens. A queue of candidates exists

to follow any failed specimens.

RIGHT: The square outline of the homogenizer is clearly evident in an EVA

specimen. The discovery experiment in (a) used a custom UV‐transmitting

test fixture (nominal concentration of 500x). Similar behavior is

anticipated in the formal experiment for a PVB specimen (b). The locally

degraded region near the center of the concentrated spot appears similar

Specimen temperature was determined using

infrared thermography. For the purpose of the work

h it i iti l th t th i t t b

y ( ),

top of South Table Mountain.

LEFT: Temperature and dose data from the OTF. The maximum

and minimum temperatures of 36°C and ‐15°C, respectively, were

observed during the first 6 months in the field. The damaging dose

of solar radiation (note logarithmic scale) is distinguished here

between “UV‐B “ (from 280‐320 nm), “UV‐A” (320‐400 nm), and

“direct“( 285‐2500 nm) wavelengths. Seasonal (winter) cloudiness

is evident in the lesser slope during the first month relative to that

encountered during the sunny summer months (the latter half of

the data). The raw dose of radiation incident on first surface of

the lens is shown in the figure. The dose at the encapsulation may

be analyzed from the measured transmittance of the CPV optical

system; its damaging effect may be further understood using the

specimen’s action  spectrum .

to those seen to precede failure in the discovery experiments.

ABOVE: The evolution of the PVB specimen from the formal study over time (1)‐(4), as photographed using a

microscope with fixed magnification stops. After the degraded region was observed at 6 months (1), the

radius of an affected region (indicated with an arrow) is seen to increase in size, even during the cold winter

months. Furthermore, separate affected regions (located adjacent to the original region) are becoming

evident with time. Damage here is thought to be aided by thermal runaway of the degraded region, based on

the increased optical absorptance of chromophore species.

The direct solar resource at the test site

(elevation of 1.79 km) includes greater UV radiation

than that found at other locations. Furthermore, the

steppe climate typically features at least 300 half‐days

of sunshine. Seasonal differences between the duration

here, it is critical that the specimen temperature be

representative of CPV module products. For example,

excessive temperature (because the cell and heat sink

are not present ) may motivate spurious failure modes.

RIGHT: The temperature of the quartz substrate is shown relative to that

of the ambient and the  poly(methyl methacrylate) (PMMA) lens. A

thermal image of the back of the module is shown in the inset. From the

temperature rise for the quartz (+40°C), the existing data suggests the

specimen temperature of 75°C during the hottest summer days (36°C).

The data for the lens indicates that its surface is cooler than the sites

where it is attached to the module. A more advanced model may be used

to account for the solar irradiance and wind.

the increased optical absorptance of chromophore species.

LEFT: Observations of silicone specimens including: (a) densification, (b) cracking, and (c) haze formation. The

densification in (a) was photographed using the method of cross‐polarization. The lack of mass loss with time

suggests the densification observed in 5 of the silicone specimens occurred during their molding. The fracture

in the harder PDMS resin in (b) occurred during a period of cold weather (when the material is least compliant

and least tough). Additional cracks have formed only during subsequent cold winter weather (not shown),

suggesting the influence of thermal misfit stress. The outline of the homogenizer  is evident in (c). Here, the

degradation is attributed to its unique formulation.

RIGHT: Optical transmittance of

the densified silicone specimen,

shown relative to a reference

specimen (composed of the same

material, but with no visible

densification) and a separate

specimen containing a

delaminated region. Densification

is clearly different from

delamination ‐ where a uniform

and magnitude of the solar radiation, however, do arise

based on the site location (latitude of 39.740 °N). A

4.2x, 2.3x, and 1.6x difference exists between the raw

dose (integral) of radiation present during the summer

and winter months based on duration, for the UV‐B, UV‐

A, and direct wavebands, respectively. Another 2.9x,

1.3x, and 1.0x difference exists between the magnitude

of radiation present during the summer and winter

months based on magnitude, for the UV‐B, UV‐A, and

direct wavebands, respectively. The damaging dose of

UV‐B radiation (which scales with both duration and

magnitude) is therefore at least 10x greater in the

summer months. For polymeric materials, radiation

induced damage may be coupled to the temperature,

further complicating seasonal differences.

ABOVE: Irradiance data (note logarithmic scale), measured on cloudless

days at the OTF . The differences in the duration and magnitude of

radiation present are readily distinguished in the figure.

optical loss results from

reflections at the glass/void

interfaces. Densification reduces

the direct transmittance,

particularly at shorter

wavelengths. Such loss would be

detrimental in a CPV module,

possibly creating a current‐limited

condition in a multijunction cell.

Future work is planned to include chemical (FTIR, RAMAN, XPS, NMR) and polymer [differential scanning

calorimetry (DSC)] analysis techniques. Traditional indicators (such as yellowness index) lack early warning (or

predictive) capability, particularly in the case of thermal runaway. It is hoped that other methods (such as

fluorescence spectrometry or chemical analysis) can provide predictive capability as well as insight into the

mechanisms of degradation. Once a sufficient knowledge base is generated (including NREL’s outdoor data as well

as that shared by CPV researchers), NREL will work towards the development of indoor accelerated test methods.

We hope to incorporate such methods into the CPV qualification standard(s) and use them to predict material

lifetime in the field.

For More Information, refer to the publications:

D.C. Miller, M.D. Kempe, C.E. Kennedy, S.R. Kurtz, “Analysis of Transmitted Optical Spectrum Enabling Accelerated Testing of Multi‐Junction CPV Designs”, Optical Engineering,

50 (1), 2010, 013003.

D.C. Miller, M.D. Kempe, K. Araki, C.E. Kennedy, S.R. Kurtz, The Durability of Polymeric Encapsulation Materials for Concentrating Photovoltaic Systems”, Proceedings of the

7th International Conference on Concentrating Photovoltaic Systems (Las Vegas, Nevada), 2011.

Summary The durability of polymeric encapsulation materials is being examined for the CPV application,

using test coupons under the optics of a CPV module product. An assortment of popular encapsulants is presently

under study. The monitoring of field conditions and diagnosis of specimen test conditions is essential to the work here.

Discovery experiments identified the importance of optical flux uniformity and control of contamination. The initial field

failures observed during the formal experiment include thermal decomposition (facilitated by thermal runaway),

fracture (facilitated by thermal misfit), and haze formation (facilitated by the formulation chemistry).  The work here

will ultimately be applied towards test standards and lifetime prediction.

Acknowledgements

The authors are grateful to: Dr. Keith Emery, Mr. Matt Muller, Dr. Daryl Myers, Dr. John Pern, Matt Beach, Christa Loux,

Tom Moricone, Marc Oddo, Bryan Price, Kent Terwilliger, and Robert Tirawat of the National Renewable Energy

Laboratory for their discussion/help with the solar spectrum, experimental methods, and optical measurements,

respectively. The authors would also like to thank Mr. Marc Gagnon of Bixby International Corp. for participation and help

with specimens and related understanding. This work was supported by the U.S. Department of Energy under Contract

No. DE‐AC36‐08GO28308 with the National Renewable Energy Laboratory.

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. NREL/PO‐5200‐50159 • February 2011