Ethylene-Vinyl Acetate Potential Problems for Photovoltaic Packaging: Preprint Page: 3 of 6
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ETHYLENE-VINYL ACETATE POTENTIAL PROBLEMS FOR PHOTOVOLTAIC
Michael D. Kempel, Gary J. Jorgensen', Kent M. Terwilliger', Tom J. McMahon', Cheryl E. Kennedy',
and Theodore T. Borek2
1 National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, CO 80401
2 Sandia National Laboratories, 1515 Eubank Blvd. SE, Albuquerque, NM 87123
Photovoltaic (PV) devices are typically encapsulated
using ethylene-vinyl acetate (EVA) to provide mechanical
support, optical coupling, electrical isolation, and
protection against environmental exposure. Under
exposure to atmospheric water and/or ultraviolet radiation,
EVA will decompose to produce acetic acid, lowering the
pH and increasing the surface corrosion rates of
embedded devices. Even though acetic acid is produced
at a very slow rate, it may not take much to catalyze
reactions that lead to rapid module deterioration. Another
consideration is that the glass transition of EVA, as
measured using dynamic mechanical analysis, begins at
temperatures of about -151C. Temperatures lower than
this can be reached for extended periods of time in some
climates. Because of increased moduli below the glass
transition temperature, a module may be more vulnerable
to damage if a mechanical load is applied by snow or wind
at low temperatures. Modules using EVA should not be
rated for use at such low temperatures without additional
low-temperature mechanical testing beyond the scope of
Polymeric encapsulant materials are used in PV
modules to provide electrical insulation and protect them
from mechanical damage and environmental corrosion. A
well bonded polymer can protect a surface by physically
preventing the accumulation of water at its bonding
interface. Water can enhance corrosion by providing a
means by which by-products can more easily diffuse away
from the surface to allow further corrosion. The presence
of counter ions (such as Cr-, Br, or acetate) similarly
enhance corrosion by facilitating the diffusion of metal ions
. Furthermore, acidic materials can catalyze the
oxidation of metals.
In some of the early work performed at the Jet
Propulsion Laboratory in the 1980s [2,3,4] a number of
encapsulant materials were investigated, and of those with
This work has been authored by an employee or employees of the Midwest Research
Institute under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy.
The United States Government retains and the publisher, by accepting the article for
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paid-up, irrevocable, worldwide license to publish or reproduce the published form of
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adequate mechanical and optical properties, EVA was
chosen because it was inexpensive. EVA continues as
the dominant encapsulant in the PV industry even though
it suffers from non-ideal mechanical and thermal
properties, a high diffusivity for water, the need for vacuum
lamination in a semi-batch manufacturing process, and the
production of acetic acid. As next-generation crystalline
silicon wafers are manufactured thinner, the mechanical
properties of EVA may not be sufficient, especially at low
service temperatures .
Because of the success of EVA with silicon-wafer
based technologies, it has often been assumed that the
generation of acetic acid is not a problem . Typical
modules with "breathable" packages should be less
affected by acetic acid than those built with impermeable
front- and back-sheets (e.g., glass), which trap
decomposition products within the package. This type of
"non-breathable" package is commonly used in thin-film
devices. This problem is exacerbated by the thinness of
the device structures, enabling small amounts of surface
corrosion to produce significant deleterious effects.
Further experimentation is necessary to evaluate the
effect of different encapsulants on the stability of thin-film
devices to determine if the higher costs of other
encapsulants can be justified by an increase in durability.
In this work, we demonstrate that the hydrolysis of
vinyl-acetate monomers results in the production of acetic
acid, which can accelerate corrosion. We further explain
how the mechanical properties of EVA are non-ideal
because of the presence of both a melting point and a
glass transition temperature (Tg) within the operating limits
of a module.
The effused gases from the thermal decomposition of
EVA were collected using a heating apparatus and an ion
chromatograph (IC) vial that contained a weighed amount
of 4.8-mM KOH. This collection solution was tested using
IC analysis to determine the acetic acid formation rate.
Dynamic mechanical analysis was performed on a TA
Instruments Ares Rheometer (equipped with an IGC
Polycold Systems Inc. cryogenic refrigeration unit model
#PGC-100 and the Ares forced convection oven) using a
rectangular torsional testing fixture. A TA Instruments
DSC Qi 000 was used for differential scanning calorimetry
(DSC). Damp heat (851C and 85% relative humidity [RH])
exposure was conducted in a Blue M environmental
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Kempe, M. D.; Jorgensen, G. J.; Terwilliger, K. M.; McMahon, T. J.; Kennedy, C. E. & Borek, T. T. Ethylene-Vinyl Acetate Potential Problems for Photovoltaic Packaging: Preprint, article, May 1, 2006; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc873283/m1/3/: accessed October 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.