Most PV bulk silicon PV modules consist of a transparent top surface, an encapsulant, a rear layer and a frame around the outer edge. In most modules, the top surface is glass, the encapsulant is EVA (ethyl vinyl acetate) and the rear layer is Tedlar, as shown below.
Front Surface Materials
The front surface of a PV module must have a high transmission in the wavelengths which can be used by the solar cells in the PV module. For silicon solar cells, the top surface must have high transmission of light in the wavelength range of 350 nm to 1200 nm. In addition, the reflection from the front surface should be low. While theoretically this reflection could be reduced by applying an anti-reflection coating to the top surface, in practice these coatings are not robust enough to withstand the conditions in which most PV systems are used. An alternative technique to reduce reflection is to "roughen" or texture the surface. However, in this case the dust and dirt is more likely to attach itself to the top surface, and less likely to be dislodged by wind or rain. These modules are not therefore "self-cleaning", and the advantages of reduced reflection are quickly outweighed by losses incurred due to increased top surface soiling.
In addition to its reflection and transmission properties, the top surface material should be impervious to water, should have good impact resistance, should be stable under prolonged UV exposure and should have a low thermal resistivity. Water or water vapor ingress into a PV module will corrode the metal contacts and interconnects, and consequently will dramatically reduce the lifetime of the PV module. In most modules the front surface is used to provide the mechanical strength and rigidity, therefore either the top surface or the rear surface must be mechanically rigid in order to support the solar cells and the wiring.
There are several choices for a top surface material including acrylic, polymers and glass. Tempered, low iron-content glass is most commonly used as it is low cost, strong, stable, highly transparent, impervious to water and gases and has good self-cleaning properties.
An encapsulant is used to provide adhesion between the solar cells, the top surface and the rear surface of the PV module. The encapsulant should be stable at elevated temperatures and high UV exposure. It should also be optically transparent and should have a low thermal resistance. EVA (ethyl vinyl acetate) is the most commonly used encapsulant material. EVA comes in thin sheets which are inserted between the solar cells and the top surface and the rear surface. This sandwich is then heated to 150 °C to polymerize the EVA and bond the module together.
The key characteristics of the rear surface of the PV module are that it must have low thermal resistance and that it must prevent the ingress of water or water vapour. In most modules, a thin polymer sheet, typically Tedlar, is used as the rear surface. Some PV modules, known as bifacial modules are designed to accept light from either the front or the rear of the solar cell. In bifacial modules both the front and the rear must be optically transparent.
A final structural component of the module is the edging or framing of the module. A conventional PV module frame is typically made of aluminium. The frame structure should be free of projections which could result in the lodgement of water, dust or other matter.
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