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

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• 作者：Xin Zhang ; Tom Scott ; Tyler Socha ; David Nielsen ; Michael Manno ; Melissa Johnson ; Yuqi Yan ; Yaroslav Losovyj ; Peter Dowben ; Eray S. Aydil ; Chris Leighton
• 刊名：ACS Applied Materials & Interfaces
• 出版年：2015
• 出版时间：July 1, 2015
• 年：2015
• 卷：7
• 期：25
• 页码：14130-14139
• ISSN：1944-8252

文摘

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 掳C) to assess phase stability, stoichiometry evolution, and their impact on transport. Bulk probes reveal the onset of pyrrhotite (Fe_{1鈭捨?/sub>S) around 400 掳C, rapidly evolving into the majority phase by 425 掳C. This is supported by X-ray photoelectron spectroscopy on (001) crystals, revealing surface Fe1鈭捨?/sub>S formation as low as 160 掳C, with rapid growth near 400 掳C. The impact on transport is dramatic, with Fe1鈭捨?/sub>S minority phases leading to a crossover from diffusive transport to hopping (due to conductive Fe1鈭捨?/sub>S nanoregions in an FeS2 matrix), followed by metallicity when Fe1鈭捨?/sub>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.}