The Layers of the Module

The array of cells must be properly encapsulated for reliable outdoor operation for more than 25 years, attending to rigidity to withstand mechanical loads, protection from weather agents and humidity, protection from impacts, electrical isolation for people safety, etc.



Figure 7.17 (a) Bowing on a solar cell after tabbing and (b) thermography of microcracks induced

by stress during stringing

Figure 7.18 Stack of materials to be laminated

The different layers the module is made of are then stacked. A common structure is sketched in Figure 7.18.

A 3- to 4-mm-thick soda-lime glass is used as a superstrate that provides mechanical rigidity and protection to the module while allowing light through. It must have low iron content or otherwise the light transmission will be low. Tempered glass must be employed to increase the resistance to impacts and for safety reasons if a module breaks.

The cell matrix is sandwiched between two layers of the encapsulant or pottant material. Except for special application such as double-glazed modules for building integration the encap – sulant is the copolymer EVA (ethylene vinyl acetate), a plastic composed of long molecules with

a backbone of carbon atoms with single covalent bonding. EVA is a thermoplastic, i. e. its shape changes under heating are reversible. It is sold in rolls of extruded film around 0.5 mm thick. Along with the polymer, the film contains: (i) curing agents whose role will be described later and (ii) UV stabilizers.

The back layer, at the non-illuminated module side, is usually a composite plastic sheet acting as a barrier for humidity and corroding species that also should provide electrical isolation for safety reasons. Typically it is formed by a laminate of three layers. The outer one is Tedlar® (commercial name of a flouropolymer by DuPont) that is an excellent barrier for external agents, however it does not provide high-voltage electrical isolation; for that a second layer made of polyester is used. For the inner part another layer of Tedlar® or a layer of EVA glued to polyester is commonly used. Some novel backsheets are based only in polyester.

Updated: August 23, 2015 — 1:28 am