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 shortcircuit current density J(SC) 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 J(SC) whereas the less reflective backsheet yields a 2.0% increase.