title: Phase Stability and Stoichiometry in Thin Film Iron Pyrite: Impact on Electronic Transport Properties.
abstract: The use of pyrite FeS2 as an earth-abundant, low-cost, nontoxic thin film photovoltaic hinges on improved understanding and control of certain physical and chemical properties. Phase stability, phase purity, stoichiometry, and defects, are central in this respect, as they are frequently implicated in poor solar cell performance. Here, phase-pure polycrystalline pyrite FeS2 films, synthesized by ex situ sulfidation, are subject to systematic reduction by vacuum annealing (to 550 °C) to assess phase stability, stoichiometry evolution, and their impact on transport. Bulk probes reveal the onset of pyrrhotite (Fe(1-δ)S) around 400 °C, rapidly evolving into the majority phase by 425 °C. This is supported by X-ray photoelectron spectroscopy on (001) crystals, revealing surface Fe(1-δ)S formation as low as 160 °C, with rapid growth near 400 °C. The impact on transport is dramatic, with Fe(1-δ)S minority phases leading to a crossover from diffusive transport to hopping (due to conductive Fe(1-δ)S nanoregions in an FeS2 matrix), followed by metallicity when Fe(1-δ)S dominates. Notably, the crossover to hopping leads to an inversion of the sign, and a large decrease in magnitude of the Hall coefficient. By tracking resistivity, magnetotransport, magnetization, and structural/chemical parameters vs annealing, we provide a detailed picture of the evolution in properties with stoichiometry. A strong propensity for S-deficient minority phase formation is found, with no wide window where S vacancies control the FeS2 carrier density. These findings have important implications for FeS2 solar cell development, emphasizing the need for (a) nanoscale chemical homogeneity, and (b) caution in interpreting carrier types and densities.
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.
authors.forename: Xin
authors.initials: X
authors.lastname: Zhang
authors.affiliation: ‡Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States.
authors.forename: Tom
authors.initials: T
authors.lastname: Scott
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.
authors.forename: Tyler
authors.initials: T
authors.lastname: Socha
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.
authors.forename: David
authors.initials: D
authors.lastname: Nielsen
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.
authors.forename: Michael
authors.initials: M
authors.lastname: Manno
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.
authors.forename: Melissa
authors.initials: M
authors.lastname: Johnson
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.
authors.forename: Yuqi
authors.initials: Y
authors.lastname: Yan
authors.affiliation: ‡Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States.|§Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States.
authors.forename: Yaroslav
authors.initials: Y
authors.lastname: Losovyj
authors.affiliation: ‡Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States.
authors.forename: Peter
authors.initials: P
authors.lastname: Dowben
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.
authors.forename: Eray S
authors.initials: ES
authors.lastname: Aydil
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.
authors.forename: Chris
authors.initials: C
authors.lastname: Leighton
keywords: charge transport; doping; phase stability; photovoltaic devices; pyrite; stoichiometry; thin films