Non-destructive optical characterization of photovoltaic modules by an integrating sphere. Part I: Mono-Si modules

The optical reflectance properties of commercial and prototype monocrystalline silicon photovoltaic (PV) modules were characterized in a non-destructive way by using an apparatus equipped with a 40-cm diameter integrating sphere. The modules showed different reflectance properties in relation to their different fabrication technologies. The lowest reflectance values, about 4% at lambda = 632.8 nm and near normal incidence, were obtained from different front structures, all containing an anti-reflection coating (ARC). Modules without ARC, on the contrary, showed total reflectances in the 6-9% interval. The total and diffuse reflectances were also measured as a function of the incident angle of a He-Ne laser beam at a fixed azimuth orientation of the incident plane. The modules with flat glass tops showed flat reflectance curves from 10 degrees to 40 degrees, whereas those with textured glass tops showed flat reflectance curves from 10 degrees to 50 degrees. in order to compare the different total reflectance curves, we introduced a 'light collection factor' for inclined light, f(IL) with respect to the normal incidence. We found a certain correlation between the light collection factor and the front structure of the modules. In particular, we established that front covers with textured glass tops collect the inclined light slightly better with respect to the front covers with flat glass tops, and then are expected also to collect slightly better the diffuse light from the sky hemisphere. Finally we found that the front covers of the mono-Si modules, as far as conditions relative to normal incidence are considered, can be optically modeled as homogeneous dielectrics with refractive index higher than that of glass (1.5) and in the interval 2.5-3.0. The precise value depends on the particular structure of the module's front cover. (C) 1999 Elsevier Science B.V. All rights reserved.