Determination and evaluation of a backsheet’s intrinsic reflectance

We measure and analyze two commercially available backsheets. One of the backsheets (AGFA’s monoblock backsheet, UniQoat) is significantly more reflective than the other (a typical Fluoropolymer/PET/Fluoropolymer backsheet). We measure their hemispherical reflectance before encapsulation in air, from which we calculate their intrinsic reflectance, and hence the reflectance at an EVA–backsheet interface after lamination. This intrinsic reflectance is considerably higher than the reflectance measured in air (e.g., the reflectance of UniQoat at 1000 nm increases from 85% to 92% after lamination), and it is this intrinsic reflectance rather than the measured reflectance that is required to accurately predict how a particular backsheet affects a module’s optical behavior. With a combination of measurements and ray tracing, we demonstrate that the calculated intrinsic reflectance is consistent with the hemispherical reflectance of glass– EVA–backsheet test samples. The ray tracing is then extended to predict how each backsheet affect a module’s short-circuit current density JSC as a function of cell spacing. We find, for example, that with a cell spacing of 4 mm, the UniQoat backsheet yields a 2.5% increase in JSC whereas the less reflective backsheet yields a 2.0% increase.