Page 203 - A Comprehensive Guide to Solar Energy Systems

Chapter 9 • Crystalline Silicon Solar Cell and Module Technology 205

The requirements for properties of encapsulants [39] are as follows:

• A low light absorption and an adapted refractive index to minimize interface
reflectance.
• A high thermal conductivity to reduce cell operating temperatures and improve
electrical yield.
• A high resistivity to ensure very low leakage currents (in accordance with IEC 61215).

In terms of PV module reliability, the encapsulant properties are critical in respect of
UV irradiation, humidity, temperature cycles, extremely low or high ambient tempera-
tures, mechanical loads, electric potential relative to ground, and so on. The encapsulant
must preserve strong adhesion to the other module components and protect the cell and
metallization from external impacts.
module manufacturers must consider material costs, processing costs, processing
time, shelf life, and quality assurance issues.
The lamination process depends on the material used. The most commonly used en-
capsulant (more than 90% of present production) is the thermoplastic, ethylene–vinyl–
acetate (eVA). It is produced as an extruded film around 0.5 mm thick. Along with the
polymer, the film contains curing agents and stabilizers whose role is in improving the
resulting properties of the encapsulating layer [40].
The next fabrication steps are lamination and curing [2,3,7]. These steps are carried
out in a laminator, a table that can be heated and furnished with a cover containing two
chambers separated with a diaphragm. Both chambers can be independently evacuated
and filled with air. The module is put in the lower chamber on the table. In the lamination
stage, both chambers are evacuated while the temperature is raised above the eVA melting
point at around 120°C. Vacuum is important to extract air to prevent voids. The melted eVA
embeds the cells and fills the space between glass, solar cells, and the rear sheet (plastic
or glass). After a few minutes, the upper chamber is filled with air so that the diaphragm
presses the laminate. Then the temperature is increased to 150°C and the curing agents
induce cross-linking of the eVA molecule chains; the eVA then acquires rubberlike proper-
ties. The curing takes between 10 and 60 min, depending on the curing agent used.
eVA is used mostly for the fabrication of modules with the glass–cells–plastic foil struc-
ture, as shown in Fig. 9.26A. For modules with the structure glass–cells–glass (Fig. 9.26B),
other materials such as polyvinyl butyral (PVB), which allows for standard lamination pro-
cesses, are to be used. Silicone has excellent properties, but it is only rarely used owing to
its high price and the need for special processing machines and techniques [41].
After lamination, the encapsulant is removed from the edges which are sealed with sili-
cone rubber and the module framed (if required). The module output contacts are then
placed in a plastic junction box, which is fixed to the back of the laminate.
To eliminate possible hot spots caused by local shading, the approach followed is to put
a diode (bypass diode) in parallel, but in opposite polarity, with a group of cells, as shown
in Fig. 9.27. The bypass diodes (usually Schottky diodes) for each substring are connected
in the junction box (see Fig. 9.27 for 18 cells in series).