Transport Evidence for Sulfur Vacancies as the Origin of Unintentional n-Type Doping in Pyrite FeS2

Chemical Bonding Induces One-Dimensional Physics in Bulk Crystal BiIr4Se8

One-dimensional (1D) systems persist as some of the most interesting because of the rich physics that emerges from constrained degrees of freedom. A desirable route to harness the properties therein is to grow bulk single crystals of a physically three-dimensional (3D) but electronically 1D compound. Most bulk compounds which approach the electronic 1D limit still field interactions across the other two crystallographic directions and, consequently, deviate from the 1D models. In this paper, we lay out chemical concepts to realize the physics of 1D models in 3D crystals. These are based on both structural and electronic arguments. We present BiIr4Se8, a bulk crystal consisting of linear Bi2+ chains within a scaffolding of IrSe6 octahedra, as a prime example. Through crystal structure analysis, density functional theory calculations, X-ray diffraction, and physical property measurements, we demonstrate the unique 1D electronic configuration in BiIr4Se8. This configuration at ambient temperature is a gapped Su-Schriefer-Heeger system, generated by way of a canonical Peierls distortion involving Bi dimerization that relieves instabilities in a 1D metallic state. At 190 K, an additional 1D charge density wave distortion emerges, which affects the Peierls distortion. The experimental evidence validates our design principles and distinguishes BiIr4Se8 among other quasi-1D bulk compounds. We thus show that it is possible to realize unique electronically 1D materials applying chemical concepts.

Crystal-chemical origins of the ultrahigh conductivity of metallic delafossites

Despite their highly anisotropic complex-oxidic nature, certain delafossite compounds (e.g., PdCoO2, PtCoO2) are the most conductive oxides known, for reasons that remain poorly understood. Their room-temperature conductivity can exceed that of Au, while their low-temperature electronic mean-free-paths reach an astonishing 20 μm. It is widely accepted that these materials must be ultrapure to achieve this, although the methods for their growth (which produce only small crystals) are not typically capable of such. Here, we report a different approach to PdCoO2 crystal growth, using chemical vapor transport methods to achieve order-of-magnitude gains in size, the highest structural qualities yet reported, and record residual resistivity ratios ( > 440). Nevertheless, detailed mass spectrometry measurements on these materials reveal that they are not ultrapure in a general sense, typically harboring 100s-of-parts-per-million impurity levels. Through quantitative crystal-chemical analyses, we resolve this apparent dichotomy, showing that the vast majority of impurities are forced to reside in the Co-O octahedral layers, leaving the conductive Pd sheets highly pure (∼1 ppm impurity concentrations). These purities are shown to be in quantitative agreement with measured residual resistivities. We thus conclude that a sublattice purification mechanism is essential to the ultrahigh low-temperature conductivity and mean-free-path of metallic delafossites.

Impact of alloying iron pyrite by ruthenium on its band gap values and its insight to photovoltaic performance

In pursuit of augmenting the band gap value of thin films composed of F e S 2 Pyrite, our study encompasses both theoretical and experimental investigations. Specifically, we sought to delve into the electronic and optical properties of F e S 2 alloyed with ruthenium, denoted as F e 1 - x R u x S 2 , where x varied across a range of values (x = 0.3966, 0.1586, 0.0496, 0.0347, 0.0106, and 0.00). Our theoretical analysis employed the Linear Muffin-Tin Orbital technique within the Atomic-Sphere approximation (LMTO-ASA) framework, focusing on the density of states. In parallel, our experimental samples were fabricated via a cost-effective and straightforward method involving the sulfuration of amorphous iron oxide thin films, which were deposited through spray pyrolysis of an aqueous solution containing FeCl3.6H2O onto heated glass substrates at 400 °C. This comprehensive investigation sheds light on the influence of alloying on the atomic structure and the optical characteristics of R u x F e 1 - x S 2 samples. Utilizing X-ray diffraction (XRD) and optical characterizations, we observed a notable widening of the band gap of F e S 2 , ranging from 0.90508 to 1.38 eV, when approximately 1.06% of the Fe atoms were replaced with ruthenium atoms (x = 0.0106 concentration of Ru). This finding holds significant implications for the potential applications of our samples in photovoltaic technologies.

Optical and Electrical Transport Evaluations of n-Type Iron Pyrite Single Crystals

Iron pyrite [cubic FeS₂ (cFeS₂)] is considered as an earth-abundant and low-cost thin-film photovoltaic material. However, the conversion efficiency of cFeS₂-based solar cells remains below 3%. To elucidate this limitation, we evaluate the optical and electrical characteristics of cFeS₂ single crystals that are grown using the flux method, thus providing us an understanding of the electron transport behavior of cFeS₂ single crystals. The oxide layer on the surface of cFeS₂, which can possibly have an influence on the electrical characteristics of cFeS₂, is removed prior to characterization via optical spectroscopy and electrical transport measurement. The optical property of cFeS₂ was found to have both indirect and direct transitions. We also observed the presence of a band tail below the conduction band. The obtained electrical transport behavior indicates that cFeS₂ bulk exhibits a high defect density and a disordered phase, thus leading to the hopping conduction mechanism. Our results will pave the way for the development of photovoltaic applications with iron pyrite.

Fabrication of Iron Pyrite Thin Films and Photovoltaic Devices by Sulfurization in Electrodeposition Method

Iron pyrite is a cheap, stable, non-toxic, and earth-abundant material that has great potential in the field of photovoltaics. Electrochemical deposition is a low-cost method, which is also suitable for large-scale preparation of iron pyrite solar cells. In this work, we prepared iron pyrite films by electrochemical deposition with thiourea and explored the effect of sulfurization on the synthesis of high-quality iron pyrite films. Upon sulfurization, the amorphous precursor film becomes crystallized iron pyrite film. Optical and electrical characterization show that its band gap is 0.89 eV, and it is an n type semiconductor with a carrier concentration of 3.01 × 10¹⁹ cm^-3. The corresponding photovoltaic device shows light response. This work suggests that sulfurization is essential in the electrochemical preparation for fabricating pure iron pyrite films, and therefore for low-cost and large-scale production of iron pyrite solar cells.

On the Mechanistic Understanding of Photovoltage Loss in Iron Pyrite Solar Cells

Considering the natural abundance, the optoelectronic properties, and the electricity production cost, iron pyrite (FeS₂ ) has a strong appeal as a solar cell material. The maximum conversion efficiency of FeS₂ solar cells demonstrated to date, however, is below 3%, which is significantly below the theoretical efficiency limit of 25%. This poor conversion efficiency is mainly the result of the poor photovoltage, which has never exceeded 0.2 V with a device having appreciable photocurrent. Several studies have explored the origin of the low photovoltage in FeS₂ solar cells, and have improved understanding of the photovoltage loss mechanisms. Fermi level pinning, surface inversion, ionization of bulk donor states, and photocarrier loss have been suggested as the underlying reasons for the photovoltage loss in FeS₂ . Given the past and more recent scientific data, together with contradictory results to some extent, it is timely to discuss these mechanisms to give an updated view of the present status and remaining challenges. Herein, the current understanding of the origin of low photovoltage in FeS₂ solar cells is critically reviewed, preceded by a succinct discussion on the electronic structure and optoelectronic properties. Finally, suggestions of a few research directions are also presented.

Voltage-induced ferromagnetism in a diamagnet

Increasingly impressive demonstrations of voltage-controlled magnetism have been achieved recently, highlighting potential for low-power data processing and storage. Magnetoionic approaches appear particularly promising, electrolytes and ionic conductors being capable of on/off control of ferromagnetism and tuning of magnetic anisotropy. A clear limitation, however, is that these devices either electrically tune a known ferromagnet or electrically induce ferromagnetism from another magnetic state, e.g., antiferromagnetic. Here, we demonstrate that ferromagnetism can be voltage-induced even from a diamagnetic (zero-spin) state suggesting that useful magnetic phases could be electrically induced in "nonmagnetic" materials. We use ionic liquid-gated diamagnetic FeS2 as a model system, showing that as little as 1 V induces a reversible insulator-metal transition by electrostatic surface inversion. Anomalous Hall measurements then reveal electrically tunable surface ferromagnetism at up to 25 K. Density functional theory-based modeling explains this in terms of Stoner ferromagnetism induced via filling of a narrow e g band.

Na-Mediated Stoichiometry Control of FeS2 Thin Films: Suppression of Nanoscale S-Deficiency and Improvement of Photoresponse

Control of the constituent phase and stoichiometry of iron pyrite (FeS₂) is a prerequisite for high-performance photovoltaic devices based on this material. If the pyrite contains sulfur-deficiency-related secondary phases which have a metallic character and a high possibility of coexistence in pyrite films, then significant carrier recombination is expected. In this work, the beneficial role of Na in suppressing the formation of nanoscale or amorphous sulfur-deficient secondary phases is reported with experimental evidence, leading to a higher phase purity for solution-processed pyrite films. The potential reduction of charge recombination via these metallic secondary phases results in significant improvements in both the photopotential and photocurrent intensity of Na-modified pyrite films compared with reference samples.

The synergistic role of double vacancies within AgGaS2 nanocrystals in carrier separation and transfer for efficient photocatalytic hydrogen evolution

The double vacancies synergistically contribute to photocatalytic activity by affecting the separation and transfer efficiency of photo-generated carriers.