228
A. Virtuani
by PET. Double layers or even single layers of polymers (so-called all-PET, all-PO
(polyolefin) backsheets)—have also been used as backsheet and are presently under
further development.
With their low permeability, polymeric backsheets protect the solar cells to a
certain extent by delaying the ingress of moisture, oxygen and other atmospheric
gases or pollutants, but are partly “breathable” allowing an atmospheric exchange
between the internal module structure and the environment. This can be an advantage,
as for example the acetic acid generated by the photo-degradation of EVA (ethyleneco-vinyl acetate) will be partly released to the external atmosphere (see Sect. 9.2.2.3).
In the presence of semiconducting and other layers, which are particularly sensitive to humidity—such as the transparent conductive oxides (TCO), used in thin film
cells and in silicon-heterojunction technology (HJT)—generally glass/glass structures, which provide a more solid packaging, are used. Alternatively, polymer backsheets containing an intermediate metal sheet (aluminium or steel) are sometimes
employed [5]. These latter are used only by a limited number of manufacturers,
but combine the lightweight properties of polymer backsheets with the reduced
permeability of glass/glass structures.
Glass/glass structures need to be symmetrical and balanced, with the rear cover
glass having similar mechanical properties (thickness, tempering, etc.) as the front
one, to avoid unnecessary stress and bending (or breakage) of the module.
9.2.2.3 Encapsulants
In a sandwich structure like that of solar modules, encapsulants serve multiple purposes: they are needed to bond different materials and layers, to hold module components (cells, interconnections, etc.) in the right position, to accommodate mechanical
stresses and to transfer mechanical solicitations trough the module structure. In addition, they isolate the solar cells electrically, and, to a certain extent, protect them from
the external environment (moisture, oxygen, other gases, etc.), preventing corrosion
and other degradation mechanisms. Besides, they improve the optical coupling of
the different materials, reducing reflection losses. A good encapsulant must provide
all these functions over a long time, even after exposure to UV radiation, humidity,
mechanical stress, and high temperatures.
In the past, several encapsulants have been deployed by the industry. These
include thermo-plastic polymers (or thermo-plastics) and thermo-setting polymers
(or elastomers). The main differences between the two classes of polymers are:
1. Thermoplastic polymers are chemically linear structures (they do not cross-ink)
while thermosetting polymers have three-dimensional cross-linked structures.
2. Thermoplastics polymer become soft and melt on heating, whereas thermosetting
polymers do not soften with heating but rather become hard, due to cross-linking;
3. Thermoplastic polymers can be reshaped, whereas thermosetting polymers
cannot be reshaped.
A. Virtuani
by PET. Double layers or even single layers of polymers (so-called all-PET, all-PO
(polyolefin) backsheets)—have also been used as backsheet and are presently under
further development.
With their low permeability, polymeric backsheets protect the solar cells to a
certain extent by delaying the ingress of moisture, oxygen and other atmospheric
gases or pollutants, but are partly “breathable” allowing an atmospheric exchange
between the internal module structure and the environment. This can be an advantage,
as for example the acetic acid generated by the photo-degradation of EVA (ethyleneco-vinyl acetate) will be partly released to the external atmosphere (see Sect. 9.2.2.3).
In the presence of semiconducting and other layers, which are particularly sensitive to humidity—such as the transparent conductive oxides (TCO), used in thin film
cells and in silicon-heterojunction technology (HJT)—generally glass/glass structures, which provide a more solid packaging, are used. Alternatively, polymer backsheets containing an intermediate metal sheet (aluminium or steel) are sometimes
employed [5]. These latter are used only by a limited number of manufacturers,
but combine the lightweight properties of polymer backsheets with the reduced
permeability of glass/glass structures.
Glass/glass structures need to be symmetrical and balanced, with the rear cover
glass having similar mechanical properties (thickness, tempering, etc.) as the front
one, to avoid unnecessary stress and bending (or breakage) of the module.
9.2.2.3 Encapsulants
In a sandwich structure like that of solar modules, encapsulants serve multiple purposes: they are needed to bond different materials and layers, to hold module components (cells, interconnections, etc.) in the right position, to accommodate mechanical
stresses and to transfer mechanical solicitations trough the module structure. In addition, they isolate the solar cells electrically, and, to a certain extent, protect them from
the external environment (moisture, oxygen, other gases, etc.), preventing corrosion
and other degradation mechanisms. Besides, they improve the optical coupling of
the different materials, reducing reflection losses. A good encapsulant must provide
all these functions over a long time, even after exposure to UV radiation, humidity,
mechanical stress, and high temperatures.
In the past, several encapsulants have been deployed by the industry. These
include thermo-plastic polymers (or thermo-plastics) and thermo-setting polymers
(or elastomers). The main differences between the two classes of polymers are:
1. Thermoplastic polymers are chemically linear structures (they do not cross-ink)
while thermosetting polymers have three-dimensional cross-linked structures.
2. Thermoplastics polymer become soft and melt on heating, whereas thermosetting
polymers do not soften with heating but rather become hard, due to cross-linking;
3. Thermoplastic polymers can be reshaped, whereas thermosetting polymers
cannot be reshaped.