Using the s‐d exchange model combined with the Green's function formalism, the effects of ionic doping on the magnetization M and the bandgap energy E g of FeS 2 in both bulk form and thin films are examined. FeS 2 is an n‐type semiconductor with an optical bandgap of approximately 0.9–0.95 eV, which is slightly smaller than the optimal value of about 1.3 eV required for photovoltaic applications. Therefore, doping with various transition metal ions, such as Co, Ni, Ru, and Cu, has been investigated. These dopants introduce different lattice strains, resulting in modified interaction constants in the doped material relative to the undoped one. For bulk FeS 2 , doping at the Fe site with Co, Ni, or Ru leads to a nonmonotonic behavior: The magnetization M reaches a maximum near a certain doping concentration x and subsequently decreases, while the bandgap energy E g exhibits a minimum around the same concentration and then increases. In contrast, for thin films with thicknesses of 30 and 10 nm, these extrema in M and E g are no longer observed. This indicates that such doped systems are promising candidates for photovoltaic applications. On the other hand, Cu doping results in a continuous reduction of E g .
Apostolov et al. (Wed,) studied this question.