Modulating the Band Gap of the FeS2 by O and Se Doping

We use the first-principles calculations to investigate whether the band gap of pyrite FeS2 can be increased by alloying with oxygen (O) or selenium (Se) atom in order to make it a more effective photovoltaic material. The results show that the conduction bands minimum (CBM) of pure FeS2 is mainly contributed by the low intensity and long trailing state of S ppσ*. Replacing sulfur (S) with isovalent O is indeed a more promising strategy for increasing the FeS2 band gap, due to the smaller radius and stronger electronegativity of oxygen. The most important is that the CBM is mainly contributed by the Fe-eg states rather than ppσ*. Significantly, the pyrite systems included S–S and O–S dimers maintain the semiconductor character, while the O–O dimer included pyrite systems change to HM at the same oxygen doping concentration for Fe32OxS64–x. However, replacing S with Se decreases the band gap, because the Se has a larger radius and similar electronegativity. The optical band gaps of the pure FeS2 and O/Se doped FeS2 systems originate mainly from electric dipole transitions between Fe-t2g states of valence bands maximum (VBM) and the bottom of the Fe-eg states in the conduction bands instead of the low intensity and long trailing S (Se) ppσ* states of the CBM.