The majority of bulk silicon PV module materials include a top surface which is transparent, an encapsulant containing the solar cells, a rear layer or back sheet and a frame surrounding the outer edge. Typically for most modules, the top surface material is glass, the encapsulant material is EVA (ethyl vinyl acetate) and the rear layer material is Tedlar.
Front Surface Module Materials
One of the module materials is the front surface of the PV module that must have a high transmission for the wavelengths that can be used by the PV module’s solar cells. For silicon solar cells, the top surface should have high transmission of the wavelength range from 350 nm to 1200 nm of incident light. Adding to that, the front surface should have low reflection. While the top surface reflection could be theoretically reduced by the application of an anti reflection coating, these coatings are not strong enough in practice to withstand the outdoor conditions in which the majority PV systems are used.
An alternative technique that can be used to reduce reflection is texturing the surface. Yet, the dust and dirt in this case are more probable to stick to the top surface, and are less likely to be removed by wind or rain. These modules as a result are not “self-cleaning”, and any advantages of reduced reflection become quickly outweighed by losses experienced because of the increased top surface soiling.
Moreover, the top surface material should be water resistant, have good impact resistance, should have a low thermal resistivity and should be steady after prolonged UV exposure. The top or the rear surface should be mechanically rigid so as to protect the solar cells and the wiring.
There are many options for top surface material such as glass, acrylic and polymers. Low iron, tempered glass is most regularly used because of low cost, impact resistance, stability, highly transparency, water resistance and has high self-cleaning properties.
An encapsulant provides adhesion between the PV module’s solar cells, top surface and rear surface. The encapsulant should have stability at long periods of high UV exposure and elevated temperatures. It must be optically transparent and have a low resistance to temperature. EVA is the most used encapsulant material. EVA is provided in thin sheets and are inserted to surround the solar cells and between the top surface and the rear surface. This formation is heated after that to 150 °C so as to polymerize the EVA material and bond the module together.
Rear Surface Module Materials
The rear surface of the PV module must have low thermal resistivity and should prevent ingress of water and water vapor. For the majority of modules, a thin sheet of polymer, Tedlar usually, is used for the rear surface. For bifacial modules both the front and the rear surfaces should be optically transparent.
The frame is the final structural component of the module. For most PV modules, the frame is made of aluminum. The most important point is that the frame structure must have no projections that can result in any collection or confinement of water, dust or other matter which would affect performance of solar cell.