Photovoltaic properties and size-pH phase stability of iron disulfide from density-functional theory

Abstract

Despite its exceptional optical absorptivity, suitable band gap, and earth abundance, the low open-circuit voltage of pyrite FeS₂ has remained the biggest challenge preventing its use in photovoltaic devices. Two widely-accepted causes are: (i) Fermi level pinning caused by intrinsic surface states that appear as gap states; (ii) presence of the polymorph marcasite. Based on density-functional theory (DFT) calculations, (i) the intrinsic (100) surface states are not gap states but located at the conduction band edge; (ii) epitaxial growth of marcasite on pyrite is thermodynamically favorable, but its band gap (from Kohn-Sham [delta]-sol method) is not less than pyrite. It is unlikely that the photovoltaic performance of pyrite is undermined by intrinsic surface states or marcasite. The stoichiometry and the ubiquitous observation of unintentional p-type conductivity of pyrite thin films are investigated via DFT defect computations. Native defects occur in low concentrations due to high formation energies, implying that pyrite is intrinsically stoichiometric. The p-type conductivity can be caused by OS defects under oxidizing conditions. Band gap engineering of pyrite is studied by alloying with non-rare-earth isovalent elements via DFT computations. We identify six MS₂ candidates that have larger band gaps than pyrite. Band gap enhancement of pyrite is observed only in the Ru and Os alloyed systems, but their incorporation into pyrite may be severely limited. All other candidate alloys exhibit large gap bowing effects due to size and/or electronegativity mismatch. The effects of particle size and pH on the relative phase stability of pyrite and marcasite polymorphs are explored. The size effect is incorporated through volume scaling of Wulff shapes. The pH effect is modeled by generalized, charged surface energies as a result of ion adsorption from the aqueous environment. Based on joint density-functional theory calculations, pyrite is unstable in highly acidic conditions due to a negative H+-adsorbed (110) surface energy, but stabilized for pH >/~ 2. Directions for future work are briefly discussed.