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Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Overview of Scientific Issues 

Involved in Selection of 

Polymers for PV Applications

Preprint

Michael Kempe

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50840

July 2011
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1

OVERVIEW OF SCIENTIFIC ISSUES INVOLVED IN SELECTION OF 

POLYMERS FOR PV APPLICATIONS

Michael Kempe

National Renewable Energy Laboratory, Golden, Colorado, USA

ABSTRACT

Encapsulant materials used in photovoltaic (PV) modules 

serve multiple purposes. They physically hold components 

in place, provide electrical insulation, reduce moisture 

ingress, optically couple superstrate materials (e.g., glass) 

to PV cells, protect components from mechanical stress by 

mechanically de-coupling components via strain relief, and 

protect materials from corrosion. To do this, encapsulants 

must adhere well to all surfaces, remain compliant, and 

transmit light after exposure to temperature, humidity, and 

UV radiation histories. Here, a brief review of some of the 

polymeric materials under consideration for PV 

applications is provided, with an explanation of some of 

their advantages and disadvantages.

TYPES OF ENCAPSULANTS

Many types of encapsulant resins have been considered 

for use in PV modules. When PV panels were first 

developed in the 1960s and 1970s, the dominant 

encapsulants were based on polydimethyl siloxane 

(PDMS) [1, 2]. This was chosen because of its exceptional 

intrinsic stability against thermal- and ultraviolet (UV) lightinduced stress [3]. However, in an effort to reduce module 

costs, alternative materials were investigated and 

developed, leading to the emergence of ethylene vinyl 

acetate (EVA) as the dominant PV encapsulant.

Recently, there has been renewed interest in using 

alternative encapsulant materials. Many of them (see Fig. 

1), including ionomer, EVA, polyvinyl butyral (PVB), and 

thermoplastic polyolefins (TPO), have a backbone 

consisting of only carbon-carbon (C-C) bonds. 

Alternatively, thermoplastic polyurethane (TPU)

formulations have nitrogen and oxygen incorporated into 

the backbone in the form of a urethane bond. The ester 

bond (R-COOR’) is susceptible to hydrolysis; however, the 

presence of hydrolytically unstable bonds in the backbone

(TPUs) is of greater concern because depolymerization 

can facilitate significant reduction in viscosity, allowing 

creep and/or delamination to occur more easily [4]. If the 

side groups of PVB or EVA become cleaved, one would 

expect to see stronger hydrogen bonding between 

polymer chains and surfaces. This can lead to 

embrittlement of polymers, however, a substantially 

greater extent of hydrolysis (compared to breaking of the 

backbone bonds in TPUs) must occur for these effects to 

be significant.

Figure 1 Structures of common PV encapsulant 

resins.

In contrast, PDMS has a backbone consisting of 

alternating atoms of Si and O. Because the silicon atom is 

much larger than oxygen or carbon atoms, there is greater 

freedom of motion for rotation and bending of Si-C sidegroup and Si-O back-bone bonds for silicone-based 

polymers compared to hydrocarbon-based polymers. This 

enhanced mobility in PDMS results in polymers with 

extremely low glass transition temperatures and with lower 

mechanical modulii (so long as the cross-link density is 

low). Additionally, the bond dissociation energy of Si-O is 

~108 kcal/mol compared to 83 kcal/mol for C-C bonds,

corresponding to photons with wavelengths of 263 and 

343 nm, respectively. The fact that no terrestrial solar 

radiation is present at 263 nm relative to that ordinarily 

Si O

CH3

CH3

n n

O

O

m

n m

O O (Na, Zn)

R

N

O

O

H n

O O HO O

O

n m o

Ionomer Thermoplastic Polyurethane

(TPU)

Polyvinyl Butyral

(PVB)

Ethylene Vinyl Acetate

(EVA)

Polydimethyl Silicone

(PDMS)

R

n m

R= -CH3 ,-(CH2)nCH3, others

Thermoplastic Polyolefin

(TPO)
2

present at 343 nm is one of the reasons for the 

exceptional UV stability of PDMS.

Typically, ionomers, TPUs, TPOs, and PVBs are 

formulated as thermoplastic (non-cross-linked) materials, 

though there is no inherent reason that they couldn’t be 

made to form cross-links and/or chemically bond to 

surfaces. For PVBs, plasticizers are also added to lower 

their mechanical modulii and to tailor their phase-transition 

temperatures. As is also summarized in [4], TPUs and 

PVBs typically have a glass transition around or below 

room temperature and are therefore in a rubbery state 

during much of their use, and susceptible to shear-induced 

flow. TPUs and PVBs are typically formulated to have a 

high viscosity at PV operating temperatures to prevent

creep [4]. Ionomers and TPOs are also typically 

thermoplastic, but often have a melt transition around 90°

to 100°C. Below the melt temperature, polyethylene 

segments are aligned forming physical cross-links whose 

formation is reversible upon heating.

To overcome concerns with polymer creep/flow at 

elevated temperatures, EVA and PDMS materials are 

typically formulated to form chemical cross-links. For 

PDMS, a Pt-based catalyst combines vinyl groups (of 

vinyl-terminated PDMS) to silane groups of a polymethylco-dimethyl siloxane (see Fig. 2). This chemistry will 

proceed at room temperature, but is significantly 

accelerated at elevated temperature. Chemical crosslinking restricts material flow to only occur when 

mechanical stresses are large enough to break chemical 

bonds. Additionally, the use of chemical cross-links 

enables more effective use of primers to promote 

adhesion at surface interfaces.

Figure 2. Schematic of curing chemistry of PDMSbased encapsulants [5].

Thus, a cross-linked system will be chemically bonded to 

surfaces, whereas thermoplastic systems must rely on a 

combination of ionic, hydrogen, and/or Van der Waals 

forces for adhesion. When water reaches an interface 

between the polymer and an inorganic material, the polar 

water molecules will compete with the less polar polymer 

at adhesion sites. If the polymer is displaced by the water, 

delamination will occur. In contrast, with a chemically 

bonded encapsulant, chemical bonds must be broken in 

addition to the physical bonds, making it easier for 

chemically bonded, cross-linked encapsulants to be 

formulated for durable interfacial adhesion.

PDMS-based materials are inherently UV and thermally 

stable, but hydrocarbon-based materials (EVA, TPU, PVB, 

and ionomer) require stabilizers to be durable. An EVA 

formulation is not just simply EVA resin, but a complex 

mixture of components. A typical EVA formulation is 

shown in Figure 3 [6, 7]. The majority of the material is the 

EVA resin. Typically a 27 wt% to 33 wt% vinyl acetate 

EVA is used to balance its characteristics, which include: a 

low glass transition, low modulus, low crystallinity/highly 

light transmittance resin, and a convenient melting 

temperature (45°C to 65°C), enabling easy melting for 

processing. EVA resins are also designed with molecular 

weight distributions and branching characteristics to 

facilitate extrusion into a film, which may minimize 

shrinkage in subsequent laminations.

About 1 to 2 wt% of an EVA film is a thermally activated 

peroxide used for cross-linking at elevated temperatures 

during lamination. The peroxide decomposes to produce 

radicals, which react with the polymer using non-specific 

chemical pathways to form cross-links. At temperatures 

above 140°C, a typical peroxide such as tertbutyl-2-

ethylhexyl-peroxycarbonate (TBEC) will decompose 

sufficiently to facilitate the cure within two minutes [8]. The 

time required to heat the polymer in a module to this 

temperature range is therefore the most significant factor 

limiting the speed of lamination.

A trialkoxy silane is used to promote adhesion between 

EVA and inorganic surfaces. The silane end tends to be 

attracted to polar surface hydroxyl groups and is able to 

react with them, creating methanol as a leaving group and

forming a covalent chemical bond in place of the hydroxyl 

group [9]. The other two alkoxy groups may further react 

with other surface groups or with other trialkoxy silane 

groups, forming a three-dimensional network that ensures

good adhesion. This interfacial structure also helps to 

passivate inorganic surfaces against corrosion by limiting 

the movement of corrosion by-products away from the 

interface.

The effects of UV radiation are mitigated by the inclusion 

of a UV absorber such as a benzotriazole or 

benzophenone. Early work on EVA formulations found an 
3

interaction between benzophenone, lupersol 101 

(peroxide), and a phenyl phosphonite that had a significant 

tendency to form chromophores [10]. These early 

formulations resulted in extreme degradation of the 

historically sizable installation of PV panels at Carrizo 

plains. Initially, the loss in power of the modules was 

attributed primarily to EVA discoloration [11], but 

subsequent analysis demonstrated that solder joint 

breakage was the more significant problem [12].

Figure 3. Example formulation of EVA for PV.

Finally, a hindered amine light stabilizer (HALS) and 

possibly a phenolic phosphonite may be added as 

antioxidants. The HALS acts to decompose peroxide 

radicals that may form due to thermal or UV exposure. In 

this process, the HALS is not consumed as opposed to the 

phenolic phosponite, which is oxidized to produce 

phosphate and phenols.

POLYMER LIGHT TRANSMITTANCE

PV encapsulants optically couple PV cells to a transparent 

superstrate such as glass; therefore, high transmittance is 

desirable. Hemispherical transmittance of light through 

encapsulant samples laminated between two pieces of 

3.2-mm-thick glass was measured to enable comparison 

of different materials, as detailed in Table 1 [13]. From 

these and similar measurements of bare glass, the photon 

transmission through a glass superstrate and 0.45 mm of 

encapsulant to a hypothetical cell interface was estimated. 

The PDMS samples had the best transmittance, about 

0.6% better than the best hydrocarbon-based materials. 

Part of this difference is attributable to the absence of UV 

absorbers in PDMS. This analysis considered only normal 

transmittance. A more thorough analysis, by McIntosh et 

al. [14], using ray tracing models and considering multiple 

reflections, non-normal incidence, and reflections off the 

backsheet between cells, estimated this difference to be 

as high as 1.5%.

Table 1. Solar photon (300 to 1100 nm) weighted 

average optical density determined from 

transmittance measurements through polymer 

samples of various thickness (1.5 to 5.5 mm) 

laminated between two pieces of 3.18-mm-thick, Cedoped, low-Fe glass [15].

UV DURABILITY

Depending on its composition, glass may block much of 

the UV-B radiation, but typically blocks very little of the 

UV-A [11, 16]. Therefore the UV stability of the 

encapsulation material used in front of the cell is 

important. Figure 4 shows the results of a highly 

accelerated stress test designed to investigate the 

possible use of non-silicone-based encapsulants in 

n

O

O

m

Ethylene Vinyl Acetate

(EVA, 96% to 98%)

N

H

R

Hinder Amine Light Stabilizer 

(HALS, 0.1% to 0.2%)

Decomposes Peroxide Radicals

N

N

N

R

R'

R"

HO

Benzoltriazole

(0.2% to 0.35%)

UV Absorber

O

O

O O

Peroxide

(1% to 2%)

Cross-Linker

Trialkoxy Silane

(0.2% to 1%)

Adhesion Promoter

Phenolic Phosphonite

(0% to 0.2%)

Peroxide Decomposer/

Radical Scavenger

R'

R P R" O

O

O

O O

O Si

O

O

AM 1.5 Solar 

Photon and xSi QE 

Weighted 

Absorptivity

Transmission 

to Cells 

through 3.18 

mm glass and 

0.45 mm 

Encapsulant

(1/mm) %

GE RTV615 0.000 ± 0.003 94.8 ± 0.3 PDMS, Addition Cure

Dow Corning 

Sylgard 184 0.002 ± 0.004 94.7 ± 0.3 PDMS, Addition Cure

Dow Corning 

527 0.004 ± 0.003 94.7 ± 0.3 PDMS Gel, Addition Cure

Polyvinyl Butyral 0.011 ± 0.005 94.3 ± 0.4

EVA 0.012 ± 0.005 94.3 ± 0.4

Thermoplastic 

Polyurethane 0.024 ± 0.004 93.8 ± 0.3

NREL 

Experimental

0.027 ± 0.006 93.7 ± 0.4

Poly α-Olefin Copolymer

Thermoplastic 

Ionomer #1 0.049 ± 0.007 92.7 ± 0.4 Copolymer of Ethylene and Methacrylic acid

DC 1199 SSL 0.064 ± 0.004 92.1 ± 0.3 PDMS, One Part Neutral 

Condensation Cure

DC 700 0.068 ± 0.004 92.0 ± 0.3 PDMS, Acetic Acid 

Condensation Cure

Thermoplastic 

Ionomer #2 0.149 ± 0.007 88.7 ± 0.4 Copolymer of Ethylene and Methacrylic acid

Encapsulant Comments
4

medium-concentration PV applications [17]. Sample 

encapsulants were laminated between two pieces of lowFe, UV-transmitting glass while monitoring the 

solar/quantum efficiency-weighted transmittance. They 

were exposed to 42 UV suns at a temperature between 

80° and 95°C. Here, none of the five different PDMS 

silicone samples demonstrated any significant loss in 

transmittance after up to 6000 h of exposure. Under the 

same conditions, the four different EVA formulations 

showed very significant degradation after only 750 to 1700 

h of exposure. This demonstrates the inherently greater 

stability of PDMS relative to EVA.

Figure 4. Solar and x-Si quantum efficiency-weighted 

transmittance of test samples exposed to 42 globalUV suns in a Xenon arc Weather-Ometer. Samples 

consist of 0.5-mm encapsulant laminated between two 

2.5-cm-square, 3.18-mm-thick, low-Fe, non-Ce glass 

samples (i.e., highly UV transmissive glass). The top 

axis corresponds to the amount of UV radition that 

would be seen with a system tracking the sun and 

utilizing only the direct spectrum.

Also important in Fig. 4 is the great variation in 

performance of the EVA formulations provided by different 

manufacturers. This is attributable to changes in either the 

type or the amount of the different additives described in 

Fig. 3. Considering the extreme conditions of this test, 

these formulations performed quite well. Similar 

experiments were also performed with PVB, TPU, and 

ionomer formulations [17]. Here PVB performed 

exceptionally poorly, TPU was comparable to EVA, and 

the ionomer was more durable than EVA. It must also be 

kept in mind that this test addressed only light 

transmittance, which is only one of several important 

characteristics such as adhesion.

Figure 5. Volume resistivity measured using 

alternating DC polarity +/- 700V. “Wet” samples were 

immersed in water at 40°C.

RESISTIVITY

The resistivity of encapsulants is relevant to electrical 

insulation, although the backsheet properties are a greater 

determining factor for a module. More importantly, 

relatively low resistance in encapsulant materials has 

been linked to electrochemical corrosion [18, 19]. The 

volume resistivity of several candidate encapsulant 

materials is shown in Fig. 5. Measurements were 

performed using the DC alternating polarity method with 

some of the samples preconditioned by soaking them in 

water at 40°C. For most materials, saturation with water

versus dry measurement did not significantly impact 

resistivity. PVB, which can absorb as much as 8% water at 

this temperature [18], was most affected by saturation with 

water. Mon et al. [18, 19] found that, for PVB and EVA, 

temperature had a much greater affect on resistivity than 

absorbed water. The EVA, TPU, and poly-α-olefin 

examined demonstrated resistivities about 100 times 

greater than PVB, and the silicone, ionomer, and EPDM 

were about 10,000 times more resistive than PVB. Mon et 

al. were also able to find good correlation between 

degradation induced by electrochemical corrosion and 

1E+10

1E+11

1E+12

1E+13

1E+14

1E+15

1E+16

1E+17

Volume Resistivity (Ohm-cm)

Polymer Resistivity

Dry

Wet
5

total leakage from cells to the frame in amorphous-Si 

based PV modules. There is a great range in the value of 

resistivity among polymers, which can be a significant 

determining factor for electrochemical corrosion 

processes.

Figure 6. Width of edge seal made from different 

materials that would be necessary to keep moisture 

below 5% of equilibrium values at a given 

temperature[20].

Figure 7. Penetration depth of moisture between glass 

plates laminated with different materials as measured 

by the oxidation of a 100 nm film of Ca [20].

MOISTURE INGRESS PREVENTION

Typical transparent encapsulant materials by themselves 

do not completely prevent water vapor ingress [20-22], but 

if they are well adhered, they will prevent the accumulation 

of liquid water providing protection against corrosion as 

well as electrical shock. Thus good adhesion is more 

important than low permeation as most module 

constructions will equilibrate with moisture in a time frame 

on the order of less than a year [22].

Even for a module construction with an impermeable 

backsheet (e.g. glass) water will still diffuse in from the 

edges. Figure 6 shows estimates of how far moisture can 

penetrate different PV polymeric materials as a function of 

temperature. Of the encapsulant materials, Ionomer #1 

has the lowest diffusivity and even if it was installed in a 

cold climate with an average effective temperature of 

200C, moisture would still penetrate to a depth of 4 or 5 

cm over the course of 20 years. Depending on the 

sensitivity of a PV material, this may not be sufficient [21].

In another set of experiments encapsulant materials and 

edge seal materials composed of polyisobutylene (PIB) 

filled with desiccants were laminated between two pieces 

of glass one of which had a 100 nm film of Ca metal 

deposited on the surface [20]. As moisture permeates the 

polymer, the Ca metal is oxidized to transparent CaO 

giving a simple visual indicator of the extent of moisture 

ingress. As shown in Fig. 7, these PIB based edge seal 

materials are an order of magnitude better than Ionomer #. 

This should enable them to restrict moisture ingress to 

less than a cm over the lifetime of a module. If a PV 

material is very sensitive to moisture, a desiccant filled PIB 

edge seal material is needed to prevent moisture ingress.

CONCLUSIONS

An encapsulant provides optical coupling of PV cells and 

protection against environmental stress. Polymers must 

perform these functions under prolonged periods of high

temperature, humidity, and UV radiation. The base 

polymer structure is the first thing to consider as it 

dominates subsequent properties. However, encapsulant 

films of the same base polymer have varying amounts and 

types of stabilization additives, resulting in different 

durabilities among manufacturers.

With the development of thin film based PV technologies, 

concerns with moisture ingress are important. Some effort 

has focused on flexible barriers with permeation rates less 

than 10-4 g/m2

/day [22], but even with an impermeable 

front- and back-sheet moisture vapor will penetrate a 

module unless extremely low permeability materials are 

used such as desiccant filled PIBs.

EVA is currently the dominant encapsulant chosen for PV 

applications, not because it has the best combination of 

properties, but because it is an economical option with an 

established history of acceptable durability. Getting new 

products into the market is challenging because there is 

1

10

100

1000

0.0027 0.0029 0.0031 0.0033 0.0035

Width of edge seal (cm)

1/K

20 year 5% Breakthrough Width

PDMS

Poly- -olefin

EVA

TPU #2

TPU #1

PVB #2

PVB #1

Acrylate

TPO #2

Ionomer #2

Ionomer #1

PET

Polyimide

PEN

PCTFE

α

850C 600C 200C

0.01

0.1

1

10

0.1 1 10 100 1000 10000 100000

Distance (cm)

850C/85% RH Exposure Time (h)

PDMS 

(0.80)

EVA 

(0.38)

PVB #1 

(0.25)

TPU #1 

(0.23)

Ionomer #1 

(0.067)

PIB #1 

(0.024)

PIB #2 

(0.018)

Material 

(K [cm/h1/2]) Typical Edge 

Seal Width
6

not room for dramatic improvements (e.g., transmittance

or price reductions), and one must balance initial cost and 

performance with the unknowns of long-term service life.

ACKNOWLEDGEMENT

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-08-GO28308 with 

the National Renewable Energy Laboratory.

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