Earth-abundant iron oxysulfide (FeSxOy) for bandgap optimization

Transition metal sulfides with small bandgap are attractive for solar cell applications since most transition metals are abundant and many can be deposited in solution. Pyrite FeS₂, with a bandgap of 0.95 eV, is one of the most desirable for solar cell applications. The problem with FeS₂ is that its bandgap is ~0.45 eV smaller than the optimum bandgap for maximum efficiency, which should be ~1.4 eV. In this paper, we propose the concept of metal oxysulfide as the approach to a low-cost Earth-abundant semiconductor with a direct bandgap of ~1.4 eV for terawatt-scale solar cells. This is because the bandgap of Fe₂O₃ is 2.2 eV. By introducing O into FeS₂, the bandgap should increase. In our experiments, we use oxidation of electrodeposited FeS_x for this purpose. FeS_x films were electrodeposited on FTO-coated glass. Post-deposition annealing was carried out in vacuum to make FeS_x films denser and more stable in air. SEM and EDX confirm that the as-grown FeS_x film is amorphous and the S/Fe ratio in the film is slightly above 1. Oxidation of the FeS_x films was performed either in a tube furnace under air or electrochemically in an electrolyte. After oxidation, the bandgap in the resultant FeS_xO_y is ~1.3 eV by electrochemical oxidation and ~1.1 eV by thermal oxidation. Further optimization is expected to produce a FeS_xO_y with a ~1.4 eV bandgap.