Most solar modules use rigid glass either as a host for the thin film coating or as an encapsulation material to provide hermetic protection. Modules based on silicon wafers usually use glass to provide the top-side encapsulation and for mounting the fragile wafers. Most thin-film solar cells and modules, such as those using CdTe, CIGS, amorphous silicon, or perovskites also use one sheet of glass to carry the thin-film and often a second glass sheet to complete the encapsulation. These glass sheets result in modules that are heavy with 1 to 6 mm of glass. In many applications, this weight is not a concern and is a trade-off to be able to use glass which is cheap and provides good protection from moisture, corrosive pollutants, UV, and oxygen.
Flexible thin-film solar modules typically use organic/inorganic barrier films for encapsulation to achieve the necessary environmental protection. These barrier films often are much thicker than the multilayer PV coating itself, even if one includes the carrier sheet used as a substrate or superstrate. Thus, available flexible modules also have considerable added mass to achieve the necessary environmental protection The need to use glass or barrier films limits the power per unit mass (specific power) that the final PV modules can achieve, even if the basic PV cell or PV film may have very high specific power.