Phase Stability and Stoichiometry in Thin Film Iron Pyrite: Impact on Electronic Transport Properties

(Figure Presented) 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-δS) 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-δS formation as low as 160°C, with rapid growth near 400°C. The impact on transport is dramatic, with Fe_1-δS minority phases leading to a crossover from diffusive transport to hopping (due to conductive Fe_1-δS nanoregions in an FeS₂ matrix), followed by metallicity when Fe_1-δS 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.