Iron chalcogenide photovoltaic absorbers -- problems and opportunities

Realizing new, efficient solar absorbers containing earth-abundant materials represents a critical element for expanding the reach of photovoltaic (PV) technologies, meeting growing energy needs. The use of Fe in PV was proposed more than 25 years ago in the form of FeS$_{2}$ pyrite. We report a concerted and integrated theoretical and experimental study that provides new insight into the problem of FeS$_{2}$. Computational results on FeS$_{2}$ reveal high formation energies for bulk point defects and small formation energies for S vacances near the surface. These findings are consistent with the formation of metallic S-deficient binary Fe-S phases at low temperatures that affect the electrical and optical properties of thin films. We have used this new understanding to propose and implement design rules for identifying new Fe-containing materials---Fe$_{2}$SiS$_{4}$ and Fe$_{2}$GeS$_{4}$-- that may circumvent the limitations of pyrite. These ternary materials are $p$-type with direct allowed optical band gaps near 1.5 eV.