Looking at the cross section of a module, there is a large disparity between the refractive index of PV cells (e. g., silicon) and packaging materials (e. g., glass and encapsulant) (Table 6.5). For instance, soda lime glass is about two units away from silicon. Most PV manufacturers make packaging choices to minimize the refractive index change across the glass-encapsulant interface.
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TABLE 6.5 Packaging Function with Corresponding Inorganic or Organic Materials and Refractive Index
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They then use an AR coating on the cell to maximize light transmission through the encapsulant-cell interface.
This same variation occurs across the interfaces of electronic assemblies, such as waveguides, optical recording devices, tunable lasers, and light-emitting diodes. In those industries, AR inorganic coatings have fallen out of favor due to difficult processing, increased material costs, and lower mechanical toughness. Instead, their approach has been to gradually change the refractive index across the interfaces to minimize loss. This approach for solar manufacturers requires polymeric encapsulants with a range of refractive indices.
Most commercial polymers have a refractive index close to one. Increasing that value closer to inorganic substances requires changing the chemical structure to a highly conjugated chain, such as polyphenylene vinylene. There has also been some industrial interest in integrating inorganic chemistry into the polymer chain. This approach, of high refractive index encap – sulants, has not found a place in current PV manufacturing because they are cost prohibitive.
