Investigation of the Sub-Bandgap Photoresponse in CuGaS2:Fe for Intermediate Band Solar Cells

Abstract

Transition-metal doped chalcopyrite thin films have been proposed as a suitable absorber material for intermediate band solar cells. In this work CuGa1-xFexS2 thin films were grown by vacuum coevaporation at a substrate temperature of 400°C with various amounts of Fe incorporated. The optical response of thin films grown on soda-lime glass was evaluated by transmittance/reflectance measurements. Photovoltaic devices were fabricated from CuGa1-xFexS2 thin films concurrently deposited on Mo-coated glass substrates using the standard chalcopyrite glass/Mo/absorber/CdS/ZnO device structure. The device characteristics of these solar cells were evaluated by current-voltage and quantum efficiency measurements. For Fe-containing CuGaS2 films distinct subgap absorption bands at 1.2 eV and 1.9 eV are detected, which increase in prominence with increasing Fe content. On the other hand, the solar cell parameters were found to deteriorate with increasing iron content indicating an increase in nonradiative recombination when high levels of iron are incorporated. However, for the lowest iron content (x=0.003) an increase in the subgap photoresponse at about 1.9 eV is observed which is attributed to a combination of sufficient intermediate band absorption and carrier collection at this dilution level.