Ann Norris, 1 Nick Powell,
James N. Cotsell, 2
1 Barry Ketola, 1
Keith R. McIntosh 2
1 Dow Corning Corporation
2 Australian National University
The photovoltaic (PV) industry is growing rapidly as the demand for cleaner energy worldwide continues to increase.
As the industry expands, it is critical that suitable material solutions are available to meet the numerous requirements including durability, performance, price, throughput and global availability.
Silicone materials have been formulated into multiple products that have a long history of successful use in a wide variety of applications and industries, such as construction and electronics industries, and are an ideal product family to meet the needs in the PV module assembly market. This paper will overview the key requirements for materials as PV encapsulants and compares some key properties of polydimethylsiloxane (PDMS) for this application and where appropriate will contrast them to organic ethylene vinyl acetate (EVA) encapsulant materials.
Some of the key properties that make
PDMS an ideal candidate for PV encapsulants include high transparency in the
UV-visible wavelength regions, very low ionic impurities, low moisture absorption, low dielectric constant and broad temperature use range. Also, silicones can be formulated to have low modulus and be stress relieving while also having excellent adhesion to the glass and cell substrates. While silicones for PV module encapsulation have been used since the
1970s, the market has historically been dominated by organic materials such as ethylene vinyl acetate (EVA). However, organic materials such as this require the addition of UV-blocking agents, and this will result in decreased transparency at low wavelengths, which in turn reduces the overall energy conversion efficiency for the PV cells and modules. Silicones have the potential to overcome the various disadvantages of using current sheet
EVA materials. With the correct design combination of materials, processing and equipment engineering, silicone encapsulants will move the PV industry toward its goal of reduced $/kWhr due to improved module efficiency, faster processing times and improved durability. Recent efforts at
Dow Corning Corporation have focused on developing new materials for both front- and back-side encapsulants for PV modules; these materials and their properties will be reviewed here.
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MODULE & PANEL CONSTRUCTION
One of the unique features of silicone materials is the variation in cure chemistries that can be used to form a cured network.[1] The most common cure system for materials used in electronics and PV device applications is the platinum catalyzed addition cure of Si-H to Si-Vinyl.
This cure system can be formulated as a one-part or two-part product, and it is a pH neutral cure system that releases no cure byproducts and therefore has no cure shrinkage. This cure system has the advantage of being able to cure at a variety of temperatures and can be formulated to be very fast curing; this can be very beneficial for high-throughput module assembly. Another unique feature to linear silicone polymers is that they can be very low viscosity prior to cure, and this allows the material to flow over and around unique features and cell architecture.
Silicones can be considered a “molecular hybrid” between inorganic glass and organic linear polymers. As shown in
Figure 1, if the Si atom is bonded only to oxygen atoms, the structure is an inorganic glass (called a Q-type Si). If one oxygen atom is substituted with an R group
(i.e., methyl, ethyl, phenyl, etc.), a resin or silsesquioxane (T-type Si) material is formed. These silsesquioxanes are more flexible than the Q-type materials. Finally, if two oxygen atoms are replaced by organic groups, a very flexible linear polymer (D-type Si) is obtained. Linear polymers will have an end group on them (Mtype Si), and those can be nonfunctional or reactive (participate in cure) groups.[2]
The increased flexibility that is found with decreasing crosslinking results in a low glass transition (Tg) of the linear polymers. The Tg of linear polydimethylsiloxane (PDMS, if all R groups are methyl units) is -120 °C. Due to the Tg, silicones also typically have a low Young’s modulus once formulated and cured, especially when compared to organic polymers. The
Figure 1 – Types of Silicone Structures Available
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Advanced Silicone Photovoltaic Encapsulants modulus in linear silicones can be quite low due to low crosslink density, and in this form silicones often function to relieve stress due to CTE mismatch between two components in many applications. Similarly, the modulus is higher in branched, tack-free resin systems. The low
Tg of PDMS also contributes to the high gas permeability of the polymers; however, because the PDMS is so hydrophobic, the equilibrium moisture content that it will retain is typically much less than corresponding organic polymers. In previous studies,[3] cured PDMS and EVA were exposed to 85 °C/85 percent RH conditions for eight weeks, and the equilibrium moisture content was measured and found to be 0.03 percent for PDMS and 0.28 percent for EVA. Because of the low moisture pickup, good adhesion and good surface wetting properties of silicones, they have been shown to protect electronic devices against moisture-induced corrosion mechanisms despite the high permeability to water vapor.[4,5]
Recent studies conducted by the
Australian National University and Dow
Corning Corporation have focused on ray trace modeling to better understand the main optical loss mechanisms (see Figure
2 for details) in PV modules. This modeling uses measured optical properties of the various layers in a PV module and the cell spectral response to quantify how the wavelength-dependent properties affect module performance. Modeling was conducted using the spectral response of three types of crystalline silicon modules: highefficiency rear contact monocrystalline; screen-printed textured monocrystalline; and screen-printed multicrystalline. An understanding of how the optical properties of the encapsulant influence these loss mechanisms was studied.[6-9] In this work, three cured silicone gel materials were included; they varied by their chemical composition, which caused the refractive index (RI) to vary. One of the key features of silicones is that the RI can vary
Figure 2 – Cross Section of Conventional PV Module and Optical Loss Mechanisms (© 2009 IEEE)
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MODULE & PANEL CONSTRUCTION from 1.38 to 1.54 (when measured at 589 nm) simply by changing the R groups attached to the silicon atom along the polymer backbone. In this work, the RI and absorption coefficient ( α ) were measured as a function of wavelength using the
Varian Cary 5000 UV-Vis spectrophotometer (see Figure 3). In this study, three silicone samples and one EVA sample were each cured between two pieces of quartz
(encapsulant was ~2 mm thick)
The absorption loss mechanisms identified for a PV module that were considered in this study and shown in Figure 2 are: (1) reflection from glass; (2) absorption in glass; (3) reflection from glass-encapsulant interface; (4) absorption in encapsulant; (5) absorption in anti-reflective coating; (6) escape reflection from encapsulant-cell interface; (7) absorption in back sheet; (8) escape reflection from back sheet.
Using the measured optical properties of the silicones and EVA, modeling was performed to quantify the optical losses of the different encapsulants for the three cell architectures. From this data, the efficiency of the modules containing the crystalline silicon solar cells were predicted to be 0.5-2.5 percent higher if silicone encapsulant is used instead of EVA
(Figure 4). The range in the efficiency was primarily due to the RI of the silicone and
Figure 3 – (a) Real refractive index; (b) absorption coefficient as a function of wavelength for STR’s
EVA, PPG’s low-iron Starphire ® glass and Dow Corning’s silicones 201 (PDMS), 203 and 205. The dotted lines in (b) show the percentage of light that is absorbed by 450 µm of material (typical thickness of EVA). (© 2009 IEEE)
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Advanced Silicone Photovoltaic Encapsulants the IQE of the cell that was modeled. The greatest benefit is seen in cells with higher efficiencies in the lower wavelength region and with higher refractive index silicones. Also the benefit is dependent upon the spectra that are modeled;[8] higher efficiencies are seen for higher blue spectra (such as those in summer as compared to winter; or cloudy rather than sunny conditions).
A key requirement for materials used in PV encapsulation is durability. The industry requires a minimum 20 years of lifetime with minimal reduction of efficiency. To predict performance, accelerated aging tests that comply with various standards such as IEC 61215, IEEE 1262 and UL 1703 are typically conducted.
Recent studies[9] compared the effects of
Figure 4 – Results from ray-tracing study plotted in terms of the module’s short-circuit current density (© 2009 IEEE)
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MODULE & PANEL CONSTRUCTION
UV (both a xenon arc lamp and an Atlas
UV2000 utilizing UVA 340 fluorescent bulbs), damp heat (85C/85 percentRH) and exposure to a 30X concentration on the optical properties of cured silicones vs.
EVA. The samples were cured between quartz substrates and the optical properties were measured as a function of time of exposure to each of these tests. It was found that the optical properties of the
PDMS remained quite stable after all exposures (a slight increase in absorption was seen after damp heat exposure) and had the least tendency to absorb moisture, which results in scattering and increased absorption. EVA had the highest tendency to absorb moisture and although it survived UV exposure, it was partially degraded by damp heat and was reduced to char after 43 days on the 30X concentration (the silicone showed very little degradation after 232 days on the tracker).
At Dow Corning Corporation, two arrays have been constructed to compare performance and durability of modules using silicone and EVA as the encapsulant. One array uses commercially available back-contact cells and one uses commercially available multicrystalline cells.
In both cases, the modules containing
EVA were purchased and the modules containing silicone were produced on a pilot line located in Freeland, Michigan.
Data of irradiance and power output has been collected for both arrays since they have been installed. The data is showing good correlation to modeled results presented previously showing about 1-2 percent increase in power output for silicones for the back-contact cells[8] and
0.5-0.75 percent increase in efficiency for the multicrystalline cells.[10] This data, although very preliminary, does demonstrate on actual arrays that the silicones do have higher power output; however, it is known that this will be dependent on many factors, such as atmospheric conditions and ambient temperature.
Silicones have the desired properties such as high transparency, high durability, low moisture uptake, good electrical insulation, flame resistance, and good adhesion, which make them excellent materials for PV encapsulants. These properties all contribute to high durability and high efficiency of PV modules. The studies have shown that silicones will increase the module efficiency (as compared to equivalent modules encapsulated with EVA) due to higher transparency in the low-wavelength region and also that they maintain higher optical transparency when compared to EVA when exposed to damp heat, UV and 30X tracker, thus showing improved long-term durability and long-term performance.
1. D.R.
Thomas, “Cross-linking of
Polydimethylsiloxanes” in Stephen J.
Clarson and J. Anthony Semlyen (eds),
Siloxane Polymers , pp. 567-611, PTR
Prentice Hall (1993).
2. W. Noll, Chemistry and Technology of
Silicones : Academic Press, New York,
1968; p. 3.
3. K.R. McIntosh, N.E. Powell, A.W.
Norris, J.N. Cotsell and B.M. Ketola,
“The effect of damp-heat and UV
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Advanced Silicone Photovoltaic Encapsulants aging tests on the optical properties of silicone and EVA encapsulants,”
Progress in Photovoltaics , in press (2010).
4. J.S. Tonge, T.H. Lane, P.A. Giwa
Agbomeirele and H.M. Klimisch, “Silbar; silicone corrosion barrier,” Proc.-
Electrochem. Soc.
, 151-159 (1989).
5. R.G. Manchke, “A Moisture Protection
Screening Test for Hybrid Circuit
Encapsulants,” Proceedings of the 31st
Electronic Components Conference , pp.
119-125 (April 1981).
6. K.R.
McIntosh, J.N.
Cotsell, J.S.
Cumpston, A.W. Norris, N.E. Powell and B.M. Ketola, “An optical comparison of silicone and EVA encapsulant for conventional silicon PV Modules: A ray-tracing study,” 34th IEEE PVSC ,
Philadelphia (2009).
7. N. Powell, B.K. Hwang, A. Norris, B.
Ketola, G.
Beaucarne and K.R.
McIntosh, “Improved Spectral
Response of Silicone and EVA
Encapsulated Photovoltaic Modules,”
35th IEEE PVSC , Hawaii (2010).
8. K.R. McIntosh, J.N. Cotsell, A.W. Norris,
N.E. Powell and B.M. Ketola, “An optical comparison of silicone and EVA encapsulants under various spectra,”
35th IEEE PVSC , Hawaii (2010).
9. K.R. McIntosh, J.N. Cotsell, J.SS.
Cumpston, A.W. Norris, N.E. Powell and B.M. Ketola, The effect of accelerated aging tests on the optical properties of silicone and EVA,” 24th EU
PVSEC , Hamburg, pp. 3475-3482 (2009).
10. J.H. Wohlgemuth, R. Clark, J. Posbic, J.
Zahler, D. Cunningham, D. Carlson,
“Reaching grid parity using BP SOLAR crystalline silicon technology,” 35th
IEEE PVSC , Hawaii (2010).
has worked at Dow Corning
Corporation for 25 years. She received her B.S.
in chemistry from University of Wisconsin at
La Crosse and her Ph.D. in materials engineering science with an emphasis on polymer characterization at Virginia Polytechnic
Institute and State University. Ann is a development scientist in the Solar Business focusing on understanding key optical properties of silicones for solar applications. Over her 25year career, she has made contributions at
Dow Corning in the areas of fluorosilicone technology, microelectronics packaging materials, photonics, LED packaging materials, and most recently in the Solar Program.
began working for Dow Corning
Corporation in 2006, with a one-year stint at
Delphi Research Labs prior to that. He received his M.S. in physics from Wayne State
University in 2006, working on dynamic light scattering to probe molecular dynamics in nematic liquid crystals. Nick is currently an
R&D physicist within the Solar Program working on optical, electrical and solid state material and device characterization and measurement method development around silicone, silicon and other materials for PV applications, modeling the effects of material property changes on systems of interest and collaborating with university and industry partners to increase the knowledge base of Dow Corning around materials and device physics to improve current, and enable future technologies through the use of silicon-based materials
joined Dow Corning in 1987.
During his career, he has held positions in manufacturing engineering, process engineer-
7 | Future Photovoltaics | September 2010

ing and application engineering, giving him a wide breadth of exposure to many products and technologies serving several industries.
Barry joined Dow Corning Solar Solutions in
2004. In this position, he has developed products and processes for the use of silicones in encapsulation of PV modules. Barry received his B.S. in chemical engineering from Michigan
Technological University.
has an honors degree in science from the Australian National University and has been working with the Centre for
Sustainable Energy Systems for 12 years. His research interests lie in concentrating PV, and he has worked in all areas of the field from manufacturing concentrator solar cells to designing and packaging solar cells into concentrator receivers. Currently James is working on the optical properties of silicon solar cells and module encapsulants, and the thermal management of cells in concentrator systems.
received his B.S. with honors in physics at the University of Sydney
(1994) and his Ph.D. in electrical engineering at the University of New South Wales
(2001). Following his Ph.D., he worked for
SunPower Corporation (2001-04) where he was a core member of the team that developed the A-300 solar cell—the most efficient silicon solar cell on today's market. In 2005,
Keith joined the Centre for Sustainable
Energy Systems at the Australian National
University, where he leads a research group investigating the optics and passivation of silicon solar cells and modules.
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MODULE & PANEL CONSTRUCTION
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