Inflexible stoichiometry in bulk pyrite FeS₂ as viewed by in situ and high-resolution X-ray diffraction

Abstract

Non-stoichiometry is considered to be one of the main problems limiting iron pyrite, FeS2, as a photovoltaic absorber material. Although some historical diffraction experiments have implied a large solubility range of FeS2−δ with δ up to 0.25, the current consensus based on calculated formation energies of intrinsic defects has lent support to line-compound behavior. Here it is shown that pyrite stoichiometry is relatively inflexible in both reductive conditions and in autogenous sulfur partial pressure, which produces samples with precise stoichiometry of FeS2 even at different Fe/S ratios. By properly standardizing in situ gas-flow X-ray diffraction measurements, no significant changes in the lattice parameter of FeS2 can be resolved, which portrays iron pyrite as prone to forming sulfur-deficient compounds, but not intrinsic defects in the manner of NiS2−δ.

Imaging the Kirkendall effect in pyrite (FeS2) thin films: Cross-sectional microstructure and chemical features

“…Besides, bulk characterization has been also the focus of some works looking to explain the general p-type behavior of synthetic pyrite films in contrast with the n-type nature of synthetic single crystals [15,[19][20][21]. Similarly, the study of the role of defects located at both surface and bulk (such as iron and sulfur vacancies) or the presence of secondary crystalline and/or amorphous phases (such as FeS x ) at the pyrite grain boundaries [18,[22][23][24] continue to be some of the subjects that still remain open.…”

Observation of an Internal p–n Junction in Pyrite FeS2 Single Crystals: Potential Origin of the Low Open Circuit Voltage in Pyrite Solar Cells

“…2,4,[6][7][8][9] While many hypotheses have emerged, a definitive explanation remains elusive, despite substantial ongoing interest in pyrite. 7,[9][10][11][12][13][14][15] Recent progress in understanding electronic transport in high quality pyrite single crystals, however, has provided perhaps the strongest clues to date. 9,16 Specifically, transport measurements on high-purity unintentionally-doped n-type pyrite single crystals have recently established a 1-3-nm-thick conductive surface layer.…”

“…Pyrite FeS 2 is a potentially ideal photovoltaic material for large-scale solar-to-electric power conversion because it is composed of nontoxic, sustainable, and inexpensive elements, has a suitable band gap (0.95 eV), and absorbs sunlight extremely strongly. , The inability to controllably dope pyrite, however, has precluded realization of pyrite p–n homojunction solar cells, and attempts at heterojunctions with various materials have consistently yielded disappointingly low efficiencies (≤3%), limited by low open-circuit voltages ( V OC ) <0.3 V. − Numerous studies, going back 30 years, have thus focused on understanding the origin of this low V OC . ,,− While many hypotheses have emerged, a definitive explanation for the low V OC , much less a solution, remains elusive, despite substantial ongoing interest in pyrite. ,− …”

Reaction Mechanism and Kinetic Model of Fe Thin Film Transformation into Monosulfides (FeS): First Step of the Fe Films Sulfuration Process into Pyrite

“…One of the most promising sulfides is pyrite (FeS 2 ), which finds potential use in thin film solar cells thanks to a convenient optical bandgap (0.9–1.0 eV , ) and high optical absorption coefficient (α ∼ 10 5 cm –1 ) , in a significant part of the solar radiation spectrum. , Moreover, intentional doping of pyrite to get controlled thin films of both n- and p-type conductivities for the fabrication of both homo- and heterostructure-based devices appears as an open and very attractive topic. − Unfortunately, pyrite photovoltaic applications are presently limited by several drawbacks of basic nature that can be divided into two main groups. First, the reduced open circuit photovoltage obtained under solar irradiation ( V OC ∼ 0.3 V), where possible causes can be related to the existence of an inversion layer at the pyrite surface or to the presence of a high density of deep donor states within the pyrite bulk. − In the second place, we find those issues related to its growth processes: type and concentration of created intrinsic point defects at both bulk and surface, − unwanted doping by uncontrolled impurities, ,− , and formation of nonpyrite sulfide phases. − ,,, It is well-known that macroscopic properties of pyrite thin films greatly depend on the specific growth technique used to obtain them and the corresponding imposed experimental parameters. For example, low sulfur-containing sulfides are frequently formed when trying to obtain pyrite films, but in different proportions and iron/sulfur ratios depending on the used experimental procedure .…”

“…17−19 In the second place, we find those issues related to its growth processes: type and concentration of created intrinsic point defects at both bulk and surface, 20−23 unwanted doping by uncontrolled impurities, 15,[22][23][24][25]16 and formation of nonpyrite sulfide phases. [19][20][21]23,26,27 It is well-known that macroscopic properties of pyrite thin films greatly depend on the specific growth technique used to obtain them and the corresponding imposed experimental parameters. For example, low sulfur-containing sulfides are frequently formed when trying to obtain pyrite films, but in different proportions and iron/sulfur ratios depending on the used experimental procedure.…”