Optical absorption enhancement in ultrathin chalcopyrite solar cells with Ag reflective back contact

Abstract

The optical performance of ultrathin solar cells based on chalcopyrite CuInS2 absorber has been studied, simulating the total absorption as a function of the absorber thickness with and without Ag reflective back contact. The structure chosen is ZnO:Al/SnS2/CuInS2/Ag/Mo/glass, using for the simulation the experimental data extracted from the layers prepared in our laboratory by evaporation (CuInS2 and SnS2) and sputtering (ZnO:Al, Ag and Mo). The main purpose has been to apply realistic input parameters (refractive index and extinction coefficient extracted from transmittance and reflectance measurements for each layer) to simulate and analyze the optical behavior of the entire device. It is found that the overall absorption of the cell with 0.2 μm-thick CuInS2 and 60 nm-thick Ag is the same as that obtained for 0.4 μm-thick CuInS2 without Ag layer. Thus, the incorporation of Ag reflective contact allows reducing the absorber layer thickness and therefore the time and cost of manufacturing chalcopyrite cells, being especially relevant for absorber thicknesses below 0.4 μm.