230
A. Virtuani
1. Water Vapor Transmission Rates (WVTR, [g/m 2 /day]): it gives a measure of
the ability of a polymer to prevent moisture ingress into the module sandwich.
Typical encapsulants do not entirely prevent water vapour ingress, but delay
it. On the other hand, a strong long-term adhesion to the solar cells, glass or
other materials is more important than low moisture ingress, as strong adhesion
will prevent the accumulation of water, avoiding corrosion and loss of electrical
insulation.
2. Electrical volume resistivity (ρ, [ cm]): it is an intrinsic property of a material quantifying how the material opposes the flow of an electrical current when
exposed to an electric field. Relatively low resistance values in encapsulant materials have been linked to high leakage currents and to the occurrence of potentialinduced degradation (PID) and electro-chemical corrosion of transparent conductive layers in thin-film technologies. The resistivity of a polymer decreases
at higher temperatures or if moisture is trapped in the polymer. Materials with a
high ρ, such as Ionomer, some polyolefins (PO) or high-ρ EVA have been shown
to prevent or strongly reduce the insurgence of PID [5, 8].
3. Optical transmittance (T, [%]): a high optical transmittance of the polymer
is required to avoid a reduction in efficiency of the solar cell. To ensure a high
optical transmittance of the polymer over time, (the latter can be seriously affected
by yellowing or browning), the UV stability of the encapsulant is critical. As
previously mentioned, in fact, glass will generally block most UV-B radiation,
but only a part of UV-A radiation.
Other relevant mechanical properties are the adhesion to a foreign substrate (glass
or foil) and the polymer stiffness. Table 9.1 provides a schematic overview of some
of these properties for the different classes of encapsulants mostly used in the solar
industry. Data are taken from the literature [9, 10] and from manufacturers’ datasheets and do not pretend to be exhaustive. Additionally, it should be noted that,
within one specific class, these parameters can vary considerably (sometimes even
over order of magnitudes, as in the case of resistivity) based on the exact formulation
of the polymer.
Table 9.1 Overview of the most relevant properties for a given class of encapsulants
Encapsulant
(thickness
0.45–0.5 mm)
WVTR
(g/m 2 /day)
Water
absorption (%)
Volume
resistivity at
23 °C ( cm)
Light
transmittance
(%)
EVA
15–25
0.2–0.3
10 13 –10 14
>92
PVB
20–25
0.4–0.5
~10 12
>92
POE/TPO
≤5
<0.1
10 14 –10 17
>92
Ionomer
≤1
<0.1
~5 × 10 16
>94
Data are taken from the literature and manufacturers’ data-sheets. Within a specific class of materials,
these parameters can vary considerably based on the exact formulation of the polymer
A. Virtuani
1. Water Vapor Transmission Rates (WVTR, [g/m 2 /day]): it gives a measure of
the ability of a polymer to prevent moisture ingress into the module sandwich.
Typical encapsulants do not entirely prevent water vapour ingress, but delay
it. On the other hand, a strong long-term adhesion to the solar cells, glass or
other materials is more important than low moisture ingress, as strong adhesion
will prevent the accumulation of water, avoiding corrosion and loss of electrical
insulation.
2. Electrical volume resistivity (ρ, [ cm]): it is an intrinsic property of a material quantifying how the material opposes the flow of an electrical current when
exposed to an electric field. Relatively low resistance values in encapsulant materials have been linked to high leakage currents and to the occurrence of potentialinduced degradation (PID) and electro-chemical corrosion of transparent conductive layers in thin-film technologies. The resistivity of a polymer decreases
at higher temperatures or if moisture is trapped in the polymer. Materials with a
high ρ, such as Ionomer, some polyolefins (PO) or high-ρ EVA have been shown
to prevent or strongly reduce the insurgence of PID [5, 8].
3. Optical transmittance (T, [%]): a high optical transmittance of the polymer
is required to avoid a reduction in efficiency of the solar cell. To ensure a high
optical transmittance of the polymer over time, (the latter can be seriously affected
by yellowing or browning), the UV stability of the encapsulant is critical. As
previously mentioned, in fact, glass will generally block most UV-B radiation,
but only a part of UV-A radiation.
Other relevant mechanical properties are the adhesion to a foreign substrate (glass
or foil) and the polymer stiffness. Table 9.1 provides a schematic overview of some
of these properties for the different classes of encapsulants mostly used in the solar
industry. Data are taken from the literature [9, 10] and from manufacturers’ datasheets and do not pretend to be exhaustive. Additionally, it should be noted that,
within one specific class, these parameters can vary considerably (sometimes even
over order of magnitudes, as in the case of resistivity) based on the exact formulation
of the polymer.
Table 9.1 Overview of the most relevant properties for a given class of encapsulants
Encapsulant
(thickness
0.45–0.5 mm)
WVTR
(g/m 2 /day)
Water
absorption (%)
Volume
resistivity at
23 °C ( cm)
Light
transmittance
(%)
EVA
15–25
0.2–0.3
10 13 –10 14
>92
PVB
20–25
0.4–0.5
~10 12
>92
POE/TPO
≤5
<0.1
10 14 –10 17
>92
Ionomer
≤1
<0.1
~5 × 10 16
>94
Data are taken from the literature and manufacturers’ data-sheets. Within a specific class of materials,
these parameters can vary considerably based on the exact formulation of the polymer