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.

Two-dimensional transition metal dichalcogenide-based counter electrodes for dye-sensitized solar cells

“…Pyrite iron disulfide (NiS 2 ) acquires unique morphological structures including nanocrystals, nanowires, nanosheets, nanocubes, and exhibits interesting electrical, photovoltaic, and catalytic properties. [211][212][213][214][215][216][217][218][219][220] An interesting comparative study was done by Shukla et al 221 In an interesting study, FeS 2 nanorod arrays were fabricated on a FTO substrate after sulfurizing FeO(OH) nanorods, and used as a CE for DSSCs. 222 The FeS 2 nanorods exhibited better electrocatalytic activity than FeS 2 films and Pt-based CEs due to more active sites, which resulted in high J SC value of the DSSCs.…”

Scientific and Technological Assessment of Iron Pyrite for Use in Solar Devices

“…2018,6,[8][9][10][11][12][13][14][15][16][17][18][19][20] 2018 Wiley-VCH Verlag GmbH &Co. KGaA,W einheim devices could be made,i np rinciple,u sing iron pyrite nanostructures.T oi mprove the photovoltage output, it would be necessary to develop methods for surface treatmenta nd intrinsic defect reduction in iron pyrite.A lso,s hunting pathways could be blocked by making better junctions, which could result in good current rectification to improve device characteristics.…”

“…[7][8][9][10][11] Despitep ossessing such desirable properties,t he success in preparing aw orking solar cell from iron pyrite has been hampered by alack of control on the chemistry of the material and by crystald efects that are believed to be responsible for the low photovoltage output from pyrite solar cells.T he crystal defects manifest as specific material conditions that include Fe-S phase impurities (troilite, greigite,p yrrhotite,e tc. ), stoichiometric deviations, [12][13][14] intrinsic bulk and surface defects, [15][16][17][18] reduced surface energy band gaps, [19] Fermi-level pinning, [20] surface-state-induced band bendinga nd ionized deep donors, [9] and surface inversion layers. [21] These issues were extensively investigated in various studies in the literature,y et they are not fully underIron pyrite (FeS 2 )h olds an enormous potential as al ow cost and non-toxic photoelectrochemicala nd energy-harvesting material owing to its interesting optical, electronic, and chemical properties along with elemental abundance.I nthis Review,l ow cost and scalable processing techniques to synthesize phase-pure pyrite thin films and nanocubes are described, and the application of this material in various energy-harvesting devices such as dye-sensitized solar cells, photodiodes,a nd heterojunctions olar cells is discussed.…”

Synthesis of Thin‐Film Metal Pyrites by an Atomic Layer Deposition Approach

“…Compared to the direct sulfurization process of thick (or even bulk) metal/oxide films, the analogous sulfurization in the ALD is only needed for the top surface atomic layer of the material, and thus intensive energy/heating, which is normally required to diffuse sulfur deep into thick films, is virtually unnecessary in the ALD case, which therefore allows the ALD process to be performed at a relatively low temperature (e.g., <200 °C). The low process temperature can be particularly beneficial for the synthesis of the transition metal sulfides, because many of these compounds are subject to losing sulfur upon high‐temperature treatment . On the other hand, there is likely a lower bound of the ALD temperature to use the H 2 S plasma, because the sulfur radicals in the plasma may combine to afford solid sulfur and condense on the sample surface at very low temperature.…”

“…The low process temperature can be particularly beneficial for the synthesis of the transition metal sulfides,b ecause many of these compounds are subjectt ol osing sulfur upon high-temperature treatment. [42] On the other hand, there is likely al ower bound of the ALD temperature to use the H 2 S plasma,because the sulfur radicals in the plasma may combine to afford solid sulfur and condense on the sample surface at very low temperature. Considering that the bulk sulfur solid has am elting point of 115 8C, the deposition temperature should not be much too lower than this point.…”