Earth-abundant iron oxysulfide (FeS_xO_y) 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.